ARM: entry: Disregard Thumb undef exception in coproc dispatch

Now that the only remaining coprocessor instructions being handled via
the dispatch in entry-armv.S are ones that only exist in a ARM (A32)
encoding, we can simplify the handling of Thumb undef exceptions, and
send them straight to the undefined instruction handlers in C code.

This also means we can drop the code that partially decodes the
instruction to decide whether it is a 16-bit or 32-bit Thumb
instruction: this is all taken care of by the undef hook.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>