[libc] Add note to use LIBC_GPU_BUILD=ON as another form

Summary:
This is a shorthand to enable GPU support so it should be listed in the
docs.

Author: jhuber6

libc/docs/gpu/building.rst

@@ -33,7 +33,8 @@ The simplest way to build the GPU libc is to use the existing LLVM runtimes
 support. This will automatically handle bootstrapping an up-to-date ``clang``
 compiler and using it to build the C library. The following CMake invocation
 will instruct it to build the ``libc`` runtime targeting both AMD and NVIDIA
-GPUs.
+GPUs. The ``LIBC_GPU_BUILD`` option can also be enabled to add the relevant
+arguments automatically.
 
 .. code-block:: sh
 
@@ -234,6 +235,10 @@ standard runtime build.
   This flag controls whether or not the libc build will generate its own
   headers. This must always be on when targeting the GPU.
 
+**LIBC_GPU_BUILD**:BOOL
+  Shorthand for enabling GPU support. Equivalent to enabling support for both
+  AMDGPU and NVPTX builds for ``libc``.
+
 **LIBC_GPU_TEST_ARCHITECTURE**:STRING
   Sets the architecture used to build the GPU tests for, such as ``gfx90a`` or
   ``sm_80`` for AMD and NVIDIA GPUs respectively. The default behavior is to