Fix the signal code on 32-bit breakpoints on arm64

When debugging 32-bit programs a debugger may insert a instruction that
will raise the undefined instruction trap. The kernel handles these
by raising a SIGTRAP, however the code was incorrect.

Fix this by using the expected TRAP_BRKPT signal code.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 67dc576bae6e624ae46962ad187e0e31fece6688)