cmd/internal/obj/mips: support NEG, avoid crash with illegal instruction

Add support of NEG{V,W} pseudo-instructions, which are translated
to a SUB instruction from R0 with proper width.

Also turn illegal instruction to UNDEF, to avoid crashing in
asmout when it tries to read the operands.

Fixes #23548.

Change-Id: I047b27559ccd9594c3dcf62ab039b636098f30a3
Reviewed-on: https://go-review.googlesource.com/89896


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>