[libc] Remove GPU string functions incompatible with C++

These functions have definitions differing between C and C++. GNU
respects the C++ definitions while the LLVM libc does not. This causes
many bugs and the current hack creates other issues. Rather than hack
around this I'd rather temporarily disable these than regress with the
integration into other offloading languages. We lose test support for
them but we should be able to re-enable these once the `libc` headers
provide these correctly.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D154850