gccrs: proc_macro: Rename from_string callback symbol

The symbol had a different convention from rust standard naming
convention (?) hence the change before adding new callback.

gcc/rust/ChangeLog:

	* expand/rust-proc-macro.cc (load_macros_array): Symbol rename.

libgrust/ChangeLog:

	* libproc_macro/proc_macro.cc (Procmacro::make_bang):
	Change symbol name.
	* libproc_macro/registration.h: Likewise.
	* libproc_macro/tokenstream.cc (TokenStream::make_tokenstream): Likewise.

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