gcc10 build issue for Arm64

[Jan Beulich <jbeulich@xxxxxxxx>]

Stefano, Julien,

since it was touching code potentially also affecting Arm, prior
to my push of a couple of commits a few minutes ago I also wanted
to build-test Arm. While as a fallback all went well with gcc
9.3, with my first (default) attempt with 10.2 I ran into

ld: prelink.o: in function `_spin_lock_cb':
/build/xen/staging-arm64/xen/common/spinlock.c:164: undefined reference to 
`__aarch64_ldadd4_acq_rel'
ld: /build/xen/staging-arm64/xen/common/spinlock.c:164: undefined reference to 
`__aarch64_ldadd4_acq_rel'
make[3]: *** [/build/xen/staging-arm64/xen/xen-syms] Error 1
make[2]: *** [/build/xen/staging-arm64/xen/xen] Error 2
make[1]: *** [install] Error 2
make: *** [install-xen] Error 2

I have no idea how recent the introduction of the issue is, but
I hope you'll be able to pinpoint the offending change (and a
possible fix) much easier than I would be.

Thanks, Jan