explow, aarch64: Fix force-Pmode-to-mem for ILP32 [PR97269]

explow, aarch64: Fix force-Pmode-to-mem for ILP32 [PR97269]

This patch fixes a mode/rtx mismatch for ILP32 targets in:

	  mem = force_const_mem (ptr_mode, imm);

where imm can be Pmode rather than ptr_mode.

The patch uses convert_memory_address to convert the Pmode address
to ptr_mode before the call.  However, immediate addresses can in
general contain unspecs, and convert_memory_address wasn't set up
to handle those.

The patch therefore adds some generic unspec handling to
convert_memory_address_addr_space_1.  As the comment says, we can add
a target hook if this behaviour turns out to be wrong for some targets.
But I think what the patch does is a strict improvement over the status
quo: without it, we would try to force the unspec into a register,
but nevertheless wrap the result in a (const ...).  That in turn
would be invalid rtl and seems bound to generate an ICE later.

I tested the explow.c part using -fstack-protector with local hacks
to force SYMBOL_FORCE_TO_MEM for UNSPEC_SALT_ADDR.

Fixes c-c++-common/torture/pr57945.c and various other tests.

gcc/
	PR target/97269
	* explow.c (convert_memory_address_addr_space_1): Handle UNSPECs
	nested in CONSTs.
	* config/aarch64/aarch64.c (aarch64_expand_mov_immediate): Use
	convert_memory_address to convert symbolic immediates to ptr_mode
	before forcing them to memory.