strub: sparc64: unbias the stack address [PR112917]

strub: sparc64: unbias the stack address [PR112917]

The stack pointer is biased by 2047 bytes on sparc64, so the range it
delimits is way off.  Unbias the addresses returned by
__builtin_stack_address (), so that the strub builtins, inlined or
not, can function correctly.  I've considered introducing a new target
macro, but using STACK_POINTER_OFFSET seems safe, and it enables the
register save areas to be scrubbed as well.

Because of the large fixed-size outgoing args area next to the
register save area on sparc, we still need __strub_leave to not
allocate its own frame, otherwise it won't be able to clear part of
the frame it should.

for  gcc/ChangeLog

	PR middle-end/112917
	* builtins.cc (expand_bultin_stack_address): Add
	STACK_POINTER_OFFSET.
	* doc/extend.texi (__builtin_stack_address): Adjust.