[libc] Add support for 'features.h' when targeting the GPU (#102037)

Summary:
`features.h` provides some information about the C library, provide this
on the GPU so external users can tell if it's the LLVM C library.

Author: jhuber6

libc/config/gpu/headers.txt

@@ -15,6 +15,7 @@ set(TARGET_PUBLIC_HEADERS
     libc.include.stdio
     libc.include.wchar
     libc.include.uchar
+    libc.include.features
 
     # Header for RPC extensions
     libc.include.gpu_rpc