[libc] Work around lack of '__has_builtin' for GPU server (#87417)

Summary:
The RPC server build for the GPU support needs to be build from the
"projects" phase of the LLVM build. That means it is built with the same
compile that LLVM supports, which currently is GCC 7.4 in most cases.
A previous patch removed the `LIBC_HAS_BUILTIN` indirection we used,
which regressed the case where we used the `libc` source externally. The
files that we need to use here are `converter.cpp` and `writer.cpp`
which currently are compatible with C++17, so there aren't issues with
the code itself. However, older GCC does not have this builtin which
makes the checks fail.

This patch just adds in a simple wrapper that allows it to correctly
ignore everything if using a compiler that doesn't support it.