[libc] Add an option to make libc only build the libc-hdrgen tool

The `libc-hdergen` tool is required for cross-builds, however some cases
can cause issues when configuring this build. This patch adds an
ovveride option `LIBC_HDRGEN_ONLY` to allow us to retain the old
(incorrect) behaviour where `libc` would not build with any other
runtimes enabled.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D151155

libc/CMakeLists.txt

@@ -28,7 +28,9 @@ if(LLVM_LIBC_FULL_BUILD OR LIBC_GPU_BUILD OR LIBC_GPU_ARCHITECTURES)
   endif()
 endif()
 
-if("libc" IN_LIST LLVM_ENABLE_RUNTIMES AND NOT LLVM_RUNTIMES_BUILD)
+option(LIBC_HDRGEN_ONLY "Only build the 'libc-hdrgen' executable" OFF)
+if(("libc" IN_LIST LLVM_ENABLE_RUNTIMES AND NOT LLVM_RUNTIMES_BUILD) OR 
+   LIBC_HDRGEN_ONLY)
   # When libc is build as part of the runtimes/bootstrap build's CMake run, we
   # only need to build the host tools to build the libc. So, we just do enough
   # to build libc-hdrgen and return.