Fix ICE with stack checking emulation at -O2

Fix ICE with stack checking emulation at -O2

On bare-metal platforms, the Ada compiler emulates stack checking (it is
required by the language and tested by ACATS) in the runtime via the
stack_check_libfunc hook of the RTL middle-end.  Calls to the function
are generated as libcalls but they now require a proper function type
at -O2 or above.

gcc/
	* explow.c: Include langhooks.h.
	(set_stack_check_libfunc): Build a proper function type.