Handle aarch64 defines seperate from arm

aarch64 is a different architecture than arm. Some
compilers might choke when both arm and aarch64
are defined.

This change separates the checks for arm and for
aarch64 in the isa_defs.h header files.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Windel Bouwman <windel@windel.nl>
Closes #10335
Closes #13151