Darwin: Rewrite host PCH support [PR 55610].

Darwin: Rewrite host PCH support [PR 55610].

We need to revise the PCH memory allocation scheme to enable
support for PIE on aarch64.  The rewrite uses a similar scheme
to the one used on Linux.

We attempt to identify VM segments for each arch/OS version that
are always available to the compiler (note this is not general,
it only needs to work for the cc1* exes).

If we cannot find the preferred segment we fall back to allowing
the kernel to supply one - this is more likely to fail when the
PCH read-in occurs (but that is trapped).

In doing this we obviate the need to unmap any part of the
compiler __DATA segment - thus fixing PR 55610.

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

	PR target/55610
	* config/host-darwin.c (TRY_EMPTY_VM_SPACE,
	SAFE_ALLOC_SIZE): New.
	(darwin_gt_pch_get_address): Rewrite to use nominated
	memory segments rather than part of the compiler __DATA
	segment.
	(darwin_gt_pch_use_address): Likewise.