arm64: print '0x' before hex values

Not making it explicit that we're printing values in hex can be
misleading when the number doesn't contain hex-only symbols (a-f). A
good example of this is print_gp_register(), where we print "(func +
offset)"; if the offset doesn't contain a-f symbols, it's not
immediately clear if that value is in decimal or hex. Using '%#' instead
of '0x%' also isn't a better option, it doesn't print '0x' if the value
is 0, and it also messes up column alignment.

Reviewed by: imp, markj
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40583