x86: honor absolute section when emitting code

x86: honor absolute section when emitting code

Various provisions exist for insns to be placed in the absolute section,
yet actually trying to do so didn't work. While data emission (of non-
zero values) is not allowed by generic code, I think this functionality
is useful for the programmer to be able to determine the size of insns.
Therefore, rather than turning the silnet failure into a verbose one,
make things mostly work; the one class of insns not supported (yet) are
branches (JMP and Jcc) with dynamically determined displacement widths.
In this one case, an error now gets reported instead of silently
ignoring the code.

Also avoid recording ISA / feature usage for insns emitted to the
absolute section.