gccrs: proc_macro: Mirror FFIString changes in C++library

Recent changes in the rust interface on some extern C function shall be
synced up in the C++ library.

libgrust/ChangeLog:

	* libproc_macro/literal.cc (Literal__from_string):
	Update to match rust interface.
	* libproc_macro/literal.h (Literal__from_string):
	Likewise.
	* libproc_macro/tokenstream.cc (TokenStream__from_string):
	Likewise.
	* libproc_macro/tokenstream.h (TokenStream__from_string):
	Likewise.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>