x86: actually implement .noopt

x86: actually implement .noopt

For quite some time we've had support for -O command line options. With
that ignoring at least .noopt isn't really a good idea.

Re-purpose the optimize-3 test for testing this directive's effect as
well.

As to the doc addition - this uses the same text as is there for the
{nooptimize} pseudo-prefix, despite me not being convinced of the "size"
part being fully accurate there (and hence also here).