arm64: Expand the use of Armv8.1-A atomics

When targeting Armv8.1 we can assume FEAT_LSE is available and can use
the atomic instructions this provides without needing to check for
support first.

Reviewed by: imp, markj, emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46159

(cherry picked from commit 7a345763f96d86eee9ab578e64311bf452e58900)