Add aarch64 pseudo help functions

Reduce code copy/paste by adding two helper functions for
aarch64_pseudo_read_value and aarch64_pseudo_write
Does not change any functionality.

gdb/
	* aarch64-tdep.c (aarch64_pseudo_read_value_1): New helper func.
	(aarch64_pseudo_write_1): Likewise.
	(aarch64_pseudo_read_value): Use helper.
	(aarch64_pseudo_write): Likewise.