riscv: use __riscv_atomic to check support A extension

GCC pre-defined some macros for detecting ISA extensions.
We should use these macros to detect ISA features.

Change-Id: I5782cdd1bf64b0161c58d789f46389dccfe44475
Signed-off-by: XiangWang <wxjstz@126.com>
Reviewed-on: https://review.coreboot.org/27300
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
1 file changed