In ELF objects the specified symbol is marked with STO_AARCH64_VARIANT_PCS.
gas/ChangeLog:
* config/tc-aarch64.c (s_variant_pcs): New function.
* doc/c-aarch64.texi: Document .variant_pcs.
* testsuite/gas/aarch64/symbol-variant_pcs-1.d: New test.
* testsuite/gas/aarch64/symbol-variant_pcs-1.s: New test.
* testsuite/gas/aarch64/symbol-variant_pcs-2.d: New test.
* testsuite/gas/aarch64/symbol-variant_pcs-2.s: New test.