cmd/internal/obj: stop using as+ALAST as an opcode
Currently, package obj reserves a range of 1<<12 opcodes for each
target architecture. E.g., mips64 has [6<<12, 7<<12).
However, because mips.ABEQ and mips.ALAST are both within that range,
the expression mips.ABEQ+mips.ALAST in turn falls (far) outside that
range around 12<<12, meaning it could theoretically collide with
another arch's opcodes.
More practically, it's a problem because 12<<12 overflows an int16,
which hampers fixing #14692. (We could also just switch to uint16 to
avoid the overflow, but that still leaves the first problem.)
As a workaround, use Michael Hudson-Doyle's solution from
https://golang.org/cl/20182 and use negative values for these variant
instructions.
Passes toolstash -cmp for GOARCH=arm and GOARCH=mips64.