aarch64: Tweak post-RA handling of CONST_INT moves [PR98136]

aarch64: Tweak post-RA handling of CONST_INT moves [PR98136]

This PR is a regression caused by r8-5967, where we replaced
a call to aarch64_internal_mov_immediate in aarch64_add_offset
with a call to aarch64_force_temporary, which in turn uses the
normal emit_move_insn{,_1} routines.

The problem is that aarch64_add_offset can be called while
outputting a thunk, where we require all instructions to be
valid without splitting.  However, the move expanders were
not splitting CONST_INT moves themselves.

I think the right fix is to make the move expanders work
even in this scenario, rather than require callers to handle
it as a special case.

gcc/
	PR target/98136
	* config/aarch64/aarch64.md (mov<mode>): Pass multi-instruction
	CONST_INTs to aarch64_expand_mov_immediate when called after RA.

gcc/testsuite/
	PR target/98136
	* g++.dg/pr98136.C: New test.