aarch64: Fix LDAPURS assembly output

... And another follow-up once I realised that the sign-extending load, of course,
needs to have strictly an X-reg as a destination for DImode extensions and a W-reg
for SImode ones.

Tested on aarch64-none-linux.

gcc/ChangeLog:

	* config/aarch64/atomics.md (*aarch64_atomic_load<ALLX:mode>_rcpc_sext):
	Use <GPI:w>  for destination format.
	* config/aarch64/iterators.md (w_sz): Delete.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/ldapr-sext.c: Adjust expected output.