arm64: Add regs_return_value() in syscall.h

This macro, regs_return_value, is used mainly for audit to record system
call's results, but may also be used in test_kprobes.c.

Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>