[libc] Add CMake cache file for the GPU build (#124589)

Summary:
This introduces libc cache files and adds one for building the GPU
support. The cache files will set defaults for these arguments which can
be overridden if the user needs to. They also serve as documentation for
how the builid is expected to look.

libc/cmake/caches/gpu.cmake

diff --git a/libc/cmake/caches/gpu.cmake b/libc/cmake/caches/gpu.cmake
new file mode 100644
index 0000000..1867db9
--- /dev/null
+++ b/libc/cmake/caches/gpu.cmake
@@ -0,0 +1,4 @@
+set(LLVM_ENABLE_PROJECTS "clang;clang-tools-extra;lld" CACHE STRING "")
+set(LLVM_RUNTIME_TARGETS default;amdgcn-amd-amdhsa;nvptx64-nvidia-cuda CACHE STRING "") 
+set(RUNTIMES_nvptx64-nvidia-cuda_LLVM_ENABLE_RUNTIMES "compiler-rt;libc" CACHE STRING "")
+set(RUNTIMES_amdgcn-amd-amdhsa_LLVM_ENABLE_RUNTIMES "compiler-rt;libc" CACHE STRING "")

libc/docs/gpu/building.rst

diff --git a/libc/docs/gpu/building.rst b/libc/docs/gpu/building.rst
index 8864357..94d3f1f 100644
--- a/libc/docs/gpu/building.rst
+++ b/libc/docs/gpu/building.rst
@@ -58,7 +58,8 @@
 ``libc`` for the GPU targets. The ``LLVM_RUNTIME_TARGETS`` sets the enabled
 targets to build, in this case we want the default target and the GPU targets.
 Note that if ``libc`` were included in ``LLVM_ENABLE_RUNTIMES`` it would build
-targeting the default host environment as well.
+targeting the default host environment as well. Alternatively, you can point
+your build towards the ``libc/cmake/caches/gpu.cmake`` cache file with ``-C``.
 
 Runtimes cross build
 --------------------