stdarch-test: various cleanups

[folkertdev]

My goal here was to make the instruction matching stricter, to prevent accepting e.g. fminnm when the test specifies the fmin instruction must be used.

But especially on x86 there are lots of cases where a prefix is (deliberately?) used.

Anyway, I also made some other cleanups. Can be reviewed commit-by-commit.

[rustbot]

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