gccrs: Make proc macro definition cdecl

gccrs: Make proc macro definition cdecl

We need to make sure proc macros have the C abi in order to be called by
the compiler with dlopen.

gcc/rust/ChangeLog:

	* backend/rust-compile-base.cc (HIRCompileBase::setup_fndecl):
	Add proc macro handlers dispatch.
	(handle_proc_macro_common): Add a function for common behavior
	between all kinds of proc macros.
	* backend/rust-compile-base.h: Add function prototypes.

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