c,c++: Fix incorrect warning with asm defined symbols

c,c++: Fix incorrect warning with asm defined symbols

Static symbols defined in assembly cause wrong "used but never defined"
warning.

static void asm_fn();
asm("%cc0:" :: ":"(&asm_fn));

This happens in C,C++ frontends before cgraph is created where the
relevant flags are located.

We can suppress these warnings with OPT_Wunused.
C,C++ frontends and cgraphunit suppressed OPT_Wunused and
OPT_Wunused_function interchangeably, so we unify suppression to
only OPT_Wunused.

	PR testsuite/123559

gcc/c/ChangeLog:

	* c-decl.cc (c_write_global_declarations_1): Check and suppress
	OPT_Wunused.
	* c-typeck.cc (build_asm_expr): Suppress OPT_Wunused.

gcc/ChangeLog:

	* cgraphunit.cc (check_global_declaration): Suppress OPT_Wunused.

gcc/cp/ChangeLog:

	* decl.cc (wrapup_namespace_globals): Check and suppress OPT_Wunused.
	* semantics.cc (finish_asm_stmt): Suppress OPT_Wunused.

gcc/testsuite/ChangeLog:

	* c-c++-common/toplevel-extended-asm-1.c: New test.