[libc] Add minimal Windows config

[caitlyncano]

Summary

A README file with procedure for building/testing LLVM libc on Windows
has also been added.

[caitlyncano]

There are a few warnings that will arise in CMake and build steps that will be resolved in followup patches.

[aeubanks]

libc/config/windows/README.md

34

I don't think we need to tell people how to build their own clang, we can tell them to get a recent clang somehow, either by downloading from https://releases.llvm.org/download.html or by building themselves (point to building clang documentation).

libc/config/windows/entrypoints.txt

19–20

do you know what character is on this newline? there's an orange exclamation mark that I haven't seen before that probably shouldn't be there. Maybe a \r?

[caitlyncano]

[libc] Updating Windows config

Removing flagged whitespace from config file and outsourcing Clang build
instructions in README.md to the official documentation online.

[aeubanks]

mostly lgtm, just a couple nits

libc/config/windows/README.md

59

maybe it's good to explicitly state that libc needs to use the recently downloaded/built clang

libc/config/windows/entrypoints.txt

19–20

now the character has moved a couple lines above

[caitlyncano]

[libc] Editing README.md

Specifying that Clang compilation is required to build LLVM libc in README.md
and fixing whitespace issue in entrypoints.txt

[aeubanks]

lgtm, thanks!

libc/config/windows/README.md

+# Building and Testing LLVM libc on Windows
+
+## Setting Up Environment
+
+To build LLVM libc on Windows, first build Clang using the following steps.
+
+1. Open Command Prompt in Windows
+2. Set TEMP and TMP to a directory. Creating this path is necessary for a
+   successful clang build.
+    1. Create tmp under your preferred directory or under `C:\src`:
+
+        ```
+        cd C:\src
+        mkdir tmp
+        ```
+
+    2. In the start menu, search for "environment variables for your account".
+       Set TEMP and TMP to `C:\src\tmp` or the corresponding path elsewhere.
+3. Download [Visual Studio Community](https://visualstudio.microsoft.com/downloads/).
+4. Install [CMake](https://cmake.org/download/) and
+   [Ninja](https://github.com/ninja-build/ninja/releases). (Optional, included
+   in Visual Studio).
+5. Load the Visual Studio environment variables using this command. This is
+   crucial as it allows you to use build tools like CMake and Ninja:
+
+    ```
+    "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsall.bat" amd64
+    ```
+
+    Note: **Rerun this command every time you open a new Command Prompt
+    window.**
+
+6. If you have not used Git before, install
+   [Git](https://git-scm.com/download/win) for Windows. Check out the LLVM
+   source tree from Github using:
+
+    ```
+    git clone https://github.com/llvm/llvm-project.git
+    ```
+
+7. Ensure you have access to Clang, either by downloading from
+   [LLVM Download](https://releases.llvm.org/download.html) or
+   [building it yourself](https://clang.llvm.org/get_started.html).
+
+## Building LLVM libc
+
+In this section, Clang will be used to compile LLVM
+libc, and finally, build and test the libc.
+
+8. Create a empty build directory in `C:\src` or your preferred directory and
+    cd to it using:
+
+    ```
+    mkdir libc-build
+    cd libc-build
+    ```
+
+9. Run the following CMake command to generate build files. LLVM libc must be built
+   by Clang, so ensure Clang is specified as the C and C++ compiler.
+
+    ```
+    cmake -G Ninja ../llvm-project/llvm -DCMAKE_C_COMPILER=C:/src/clang-build/bin/clang-cl.exe -DCMAKE_CXX_COMPILER=C:/src/clang-build/bin/clang-cl.exe  -DLLVM_TARGETS_TO_BUILD=X86 -DLLVM_FORCE_BUILD_RUNTIME=libc -DLLVM_ENABLE_PROJECTS=libc -DLLVM_NATIVE_ARCH=x86_64 -DLLVM_HOST_TRIPLE=x86_64-window-x86-gnu
+    ```
+
+10. Build LLVM libc using:
+
+    ```
+    ninja llvmlibc
+
+    ```
+
+11. Run tests using:
+
+    ```
+    ninja checklibc
+    ```

libc/config/windows/entrypoints.txt

+set(TARGET_LIBC_ENTRYPOINTS
+    # ctype.h entrypoints
+    libc.src.ctype.isalnum
+    libc.src.ctype.isalpha
+    libc.src.ctype.isascii
+    libc.src.ctype.isblank
+    libc.src.ctype.iscntrl
+    libc.src.ctype.isdigit
+    libc.src.ctype.isgraph
+    libc.src.ctype.islower
+    libc.src.ctype.isprint
+    libc.src.ctype.ispunct
+    libc.src.ctype.isspace
+    libc.src.ctype.isupper
+    libc.src.ctype.isxdigit
+    libc.src.ctype.toascii
+    libc.src.ctype.tolower
+    libc.src.ctype.toupper
+    # string.h entrypoints
+    libc.src.string.bzero
+    libc.src.string.memchr
+    libc.src.string.memcmp
+    libc.src.string.memcpy
+    libc.src.string.memmove
+    libc.src.string.memset
+    libc.src.string.memrchr
+    libc.src.string.strcat
+    libc.src.string.strchr
+    libc.src.string.strcpy
+    libc.src.string.strcmp
+    libc.src.string.strcspn
+    libc.src.string.strlen
+    libc.src.string.strncpy
+    libc.src.string.strnlen
+    libc.src.string.strpbrk
+    libc.src.string.strrchr
+    libc.src.string.strspn
+    libc.src.string.strstr
+    libc.src.string.strtok
+    libc.src.string.strtok_r
+)
+set(TARGET_LLVMLIBC_ENTRYPOINTS
+  ${TARGET_LIBC_ENTRYPOINTS}
+)