target/xtensa: statically allocate xtensa_insnbufs in DisasContext

target/xtensa: statically allocate xtensa_insnbufs in DisasContext

Rather than dynamically allocate, and risk failing to free
when we longjmp out of the translator, allocate the maximum
buffer size based on the maximum supported instruction length.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Richard Henderson <richard.henderson@linaro.org>