Fix bad libbxo format strings in jls

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