The fucomi* opcodes were only introduced for the Pentium Pro.
They do not exist for an MMX Pentium.  Use the fucom* instructions
instead and move the condition codes from the fp flags register to
the integer flags register explicitly.

The use of fucomi* opcodes in ggen.go was introduced in 1.5 (CL 8738).
The bad ops were generated for 64-bit floating-point comparisons.

The use of fucomi* opcodes in gsubr.go dates back to at least 1.1.
The bad ops were generated for float{32,64} to uint64 conversions.

Fixes #13923

Change-Id: I5290599f5edea8abf8fb18036f44fa78bd1fc9e6
Reviewed-on: https://go-review.googlesource.com/18590Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>