target-tilegx: Decode ill pseudo-instructions

Notice raise and bpt, decoding the constants embedded in the
nop addil instruction in the x0 slot.

[rth: Generalize TILEGX_EXCP_OPCODE_ILL to TILEGX_EXCP_SIGNAL.
Drop validation of signal values.]

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Message-Id: <1443243635-4886-1-git-send-email-gang.chen.5i5j@gmail.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
