[Docs] Mention LLVM_EXPERIMENTAL_TARGETS_TO_BUILD variable in CMake.rst

[xgupta]

Summary

Beginners might not aware of this variable and wanted to try a new experimental target.

Although this variable mention in Writing a Backend Documentation. But it becomes easy to search when listed in cmake.rst doc where most variables are listed.

[thakis]

Can we add some words on how to find valid values to set this to?

[xgupta]

Actually, I don't know/find a way to list all experimental targets, @myhsu might aware of such an option as he is actively working on experimental target M68k.

[myhsu]

In D100729#2697609, @thakis wrote:

Can we add some words on how to find valid values to set this to?

IIRC LLVM doesn't have a list of experimental targets in any of the CMake files (LLVM only has a list of targets built by default). The only way to find out is subtracting all folder names under llvm/lib/Target by the value of LLVM_ALL_TARGETS cmake variable inside llvm/CMakeLists.txt

[xgupta]

Yeah, I also think this is the only way to know experimental targets.

Please accept the revision if it looks good.

[myhsu]

llvm/docs/CMake.rst

232

I don't think we should mention LLVM_ALL_TARGETS here since it's an internal variable.

[xgupta]

Completely agree, remove the LLVM_ALL_TARGETS variable from the wording.

[myhsu]

LGTM

llvm/docs/CMake.rst

 LLVM-specific variables
 -----------------------
 
 **LLVM_TARGETS_TO_BUILD**:STRING
   Semicolon-separated list of targets to build, or *all* for building all
   targets. Case-sensitive. Defaults to *all*. Example:
   ``-DLLVM_TARGETS_TO_BUILD="X86;PowerPC"``.
 
+**LLVM_EXPERIMENTAL_TARGETS_TO_BUILD**:STRING
+  Semicolon-separated list of experimental targets to build and linked into 
+  llvm. This will build the experimental target without needing it to add to the 
+  list of all the targets available in the LLVM's main CMakeLists.txt.
+
 **LLVM_BUILD_TOOLS**:BOOL
   Build LLVM tools. Defaults to ON. Targets for building each tool are generated
   in any case. You can build a tool separately by invoking its target. For
   example, you can build *llvm-as* with a Makefile-based system by executing *make
   llvm-as* at the root of your build directory.
 
 **LLVM_INCLUDE_TOOLS**:BOOL
   Generate build targets for the LLVM tools. Defaults to ON. You can use this