aarch64: override default elf .set handling in gas

aarch64: override default elf .set handling in gas

Allow st_other values such as STO_AARCH64_VARIANT_PCS to be set for alias
symbols independently.  This is needed for ifunc symbols which are
aliased to the resolver using .set and don't expect resolver attributes
to override the ifunc symbol attributes.  This means .variant_pcs must be
added explicitly to aliases.

gas/ChangeLog:

	* config/tc-aarch64.c (aarch64_elf_copy_symbol_attributes): Define.
	* config/tc-aarch64.h (aarch64_elf_copy_symbol_attributes): Declare.
	(OBJ_COPY_SYMBOL_ATTRIBUTES): Define.
	* testsuite/gas/aarch64/symbol-variant_pcs-3.d: New test.
	* testsuite/gas/aarch64/symbol-variant_pcs-3.s: New test.