[libc][doc] Add macros guidelines

[gchatelet]

Summary

This is a first follow up on the libc tuning RFC
https://discourse.llvm.org/t/rfc-llvm-libc-tuning/67980

Once we agree on the format. I'll land a couple of patches to match the guidelines.

libc/docs/code_style.rst

#. **const and constexpr variables** - They use the capitlized
    ``SNAKE_CASE`` irrespective of whether they are local or global.
 #. **Function and methods** - They use the ``snake_case`` style like the
    non-const variables.
 #. **Internal type names** - These are types which are interal to the libc
    implementation. They use the ``CaptilizedCamelCase`` style.
 #. **Public names** - These are the names as prescribed by the standards and
    will follow the style as prescribed by the standards.
 
+Macro style
+===========
+
+We define two kinds of macros: **code defined** and **build defined** macros.
+
+#. **Build defined** macros are generated by `CMake` or `Bazel` and are passed
+   down to the compiler with the ``-D`` command line flag. They start with the
+   ``LIBC_COPT_`` prefix. They are used to tune the behavior of the libc.
+   They either denote an action or define a constant.
+
+#. **Code defined** macros are defined within the ``src/__support/macros``
+   folder. They all start with the ``LIBC_`` prefix. They are of two kinds
+
+   * **Properties** - Build related properties like used compiler, target
+     architecture or enabled CPU features defined by introspecting compiler
+     defined preprocessor defininitions. e.g., ``LIBC_TARGET_IS_ARM``,
+     ``LIBC_TARGET_HAS_AVX2``, ``LIBC_COMPILER_IS_CLANG``, ...
+   * **Attributes** - Compiler agnostic attributes or functions to handle
+     specific operations. e.g., ``LIBC_INLINE``, ``LIBC_NO_LOOP_UNROLL``,
+     ``LIBC_LIKELY``, ``LIBC_INLINE_ASM``.
+
 Inline functions defined in header files
 ========================================
 
 When defining functions inline in header files, we follow certain rules:
 
 #. The functions should not be given file-static linkage. There can be class
    static methods defined inline however.
 #. Instead of using the ``inline`` keyword, they should be tagged with the