Darwin, PCH: Rework hooks for relocatable implementation [PR71934].

Darwin, PCH: Rework hooks for relocatable implementation [PR71934].

Now we have a relocatable PCH implementation we can revise the
hooks that find and use the mmapped memory.  Specifically, this
removes the extra checking and diagnostic output for cases that
were likely to fail in a non-relocatable scenario.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
gcc/ChangeLog:

	PR pch/71934
	* config/host-darwin.c (SAFE_ALLOC_SIZE): Remove.
	(darwin_gt_pch_get_address): Rework for relocatable PCH.
	(darwin_gt_pch_use_address): Likewise.