cmd/compile: stop using fucomi* ops for 387 builds
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.