cmd/vet: add s390x support

[josharian]

cmd/vet has support for checking assembly. It requires a few details about the architecture. See asmdecl.go. It'd be good to add s390x to it. Unless s390x is very unusual, it should be an easy two-line addition to the block of asmArch* vars.

/cc @billotosyr @mundaym

[mundaym]

This is a duplicate of #15454. The fix is waiting on a test.

[minux]

Merged into #15454.