more xscale cleanup (mostly removing JTAG hooks)

Streamline/shrink some needless JTAG stuff:

 - Use #defines for the JTAG instructions; they can't ever change
 - Remove an unused (!) shadow of tap->ir_length
 - Stop using a copy of target->tap
 - Don't bother saving the variant after sanity checking ir_length

Also, make target_create() work as on other targets:  build the
register cache later, making init_target() no longer be a NOP.
Handle malloc failure; remove a comment that was obsoleted by the
not-so-new target syntax.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>