grub-mkimage: Only check aarch64 relocations when built for aarch64
Coverity flagged the switch checks for R_AARCH64_* as being logically
dead code, since it could never happen on x86 due to the masking of the
values earlier in the code.
A check for building on __arm__ (which gcc and clang define) and for
MKIMAGE_ELF64 (which GRUB defines) has been added to avoid this dead
code being built in.
Fixes: CID 158599 Signed-off-by: Darren Kenny <darren.kenny@oracle.com> Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>