target/i386: clarify (un)signedness of immediates from 0F3Ah opcodes

target/i386: clarify (un)signedness of immediates from 0F3Ah opcodes

Three-byte opcodes from the 0F3Ah area all have an immediate byte which
is usually unsigned.  Clarify in the helper code that it is unsigned;
the new decoder treats immediates as signed by default, and seeing
an intN_t in the prototype might give the wrong impression that one
can use decode->immediate directly.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>