x86: clean up after removal of support for gcc <= 2.8.1

x86: clean up after removal of support for gcc <= 2.8.1

At the very least a comment in process_operands() is stale. Beyond that
there are effectively two options:
1) It is possible that FADDP and FMULP were mistakenly not marked as
   being in need of dealing with the compiler anomaly, and hence the
   respective templates weren't removed at the time when they should
   have been.
2) It is also possible that there are indeed uses known beyond compiler
   generated output for these two commutative opcodes, and hence the
   templates need to stay.
To be on the safe side assume 2: Update the comment and fold the
templates into their "normal" ones (utilizing D), adjusting consuming
code accordingly.

For FMULP also add a comment paralleling a similar one FADDP has.