module/icp/asm-arm/sha2: auto detect __ARM_ARCH

This patch uses __ARM_ARCH set by compiler (both
GCC and Clang have this) whenever possible instead
of hardcoding it to 7. This change allows code to
compile on earlier ARM architectures such as armv5te.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Shengqi Chen <harry-chen@outlook.com>
Closes #15557