aarch64: handle STO_AARCH64_VARIANT_PCS in bfd

aarch64: handle STO_AARCH64_VARIANT_PCS in bfd

Propagate STO_AARCH64_VARIANT_PCS st_other attribute to the output and
add DT_AARCH64_VARIANT_PCS dynamic tag if necessary.

Mismatching attributes are not diagnosed.

bfd/ChangeLog:

	* elfnn-aarch64.c (elfNN_aarch64_merge_symbol_attribute): New function.
	(struct elf_aarch64_link_hash_table): Add variant_pcs member.
	(elfNN_aarch64_allocate_dynrelocs): Update variant_pcs.
	(elfNN_aarch64_size_dynamic_sections): Add DT_AARCH64_VARIANT_PCS.
	(elf_backend_merge_symbol_attribute): Define.

ld/ChangeLog:

	* testsuite/ld-aarch64/aarch64-elf.exp: Add new tests.
	* testsuite/ld-aarch64/variant_pcs-1.s: New asm for tests.
	* testsuite/ld-aarch64/variant_pcs-2.s: New asm for tests.
	* testsuite/ld-aarch64/variant_pcs-now.d: New test.
	* testsuite/ld-aarch64/variant_pcs-r.d: New test.
	* testsuite/ld-aarch64/variant_pcs-shared.d: New test.
	* testsuite/ld-aarch64/variant_pcs.ld: New linker script for tests.