arm64: fix mask in atomic_testand{set,clear}_64

These macros generate both the 32- and 64-bit ops, but the mask was hard
coded for 32-bit ops, causing the 64-bit ops always to affect only the
low 32 bits.

PR: 252324
Reported by: gbe, mmel
Reviewed by: markj, mmel
Tested by: mmel, rwatson
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D27886