gcn: Add __builtin_gcn_{get_stack_limit,first_call_this_thread_p}

The new builtins have been added for newlib to reduce dependency on
compiler-internal implementation choices of GCC in newlibs' getreent.c.

gcc/ChangeLog:

	* config/gcn/gcn-builtins.def (FIRST_CALL_THIS_THREAD_P,
	GET_STACK_LIMIT): Add new builtins.
	* config/gcn/gcn.cc (gcn_expand_builtin_1): Expand them.
	* config/gcn/gcn.md (prologue_use): Add "register_operand" as
	arg to match_operand.
	(prologue_use_di): New; DI insn_and_split variant of the former.

Co-Authored-By: Andrew Stubbs <ams@codesourcery.com>