xen/arm64: force gcc 10+ to always inline generic atomics helpers

xen/arm64: force gcc 10+ to always inline generic atomics helpers

Recent versions of gcc (at least 10.x) will not inline generic atomics
helpers by default. Instead they will expect the software to either link
with libatomic.so or implement the helpers, which would result in

undefined reference to `__aarch64_ldadd4_acq_rel'

for us (not having any local implementation).

To keep the previous behavior, force gcc to always inline the generic
atomics helpers.

Long term we probably want to avoid relying on gcc atomics helpers as
this doesn't allow us to switch between LSE and LL/SC atomics.

Suggested-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>