Revert "target/arm: Use unallocated_encoding for aarch32"

Revert "target/arm: Use unallocated_encoding for aarch32"

This reverts commit 3cb36637157088892e9e33ddb1034bffd1251d3b.

Despite the fact that the text for the call to gen_exception_insn
is identical for aarch64 and aarch32, the implementation inside
gen_exception_insn is totally different.

This fixes exceptions raised from aarch64.

Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Message-id: 20190826151536.6771-2-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>