Fix bad libbxo format strings in jls

The existing format string for the empty case was trying to read varargs
values that weren't passed to xo_emit. This appears to work on x86 (since
the next argument is probably a pointer an empty string), but for CHERI
we can bound variadic arguments and detect a read past the end.

While touching these lines also use the libxo 'a' modifier to avoid having to
construct the libxo format string using asprintf.

Found by: CHERI
Reviewed By: allanjude
Differential Revision: https://reviews.freebsd.org/D26885