arm64: Handle 32bits breakpoint exception.

A different exception is raised when we hit a 32bits breakpoint, rather than
a 64bits one, so handle those as well when COMPAT_FREEBSD32 is defined.
This should fix SIGBUS at least when using breakpoints with thumb2 code.

PR: 256468
MFC After: 1 week

(cherry picked from commit 2734050154927eaa63d3b65de5d46d05569b3a5b)