cmd/compile: some x/sys versions no longer build due to "go:linkname must refer to declared function or variable" [1.19 backport]

[gopherbot]

@mdempsky requested issue #55889 to be considered for backport to the next 1.19 minor release.

@gopherbot Please backport to Go 1.18 and 1.19. This was is regression in the Go compiler, which broke building x/sys for GOOS=darwin prior to https://go.dev/cl/274573 (golang/sys@69691e4).

[gopherbot]

Change https://go.dev/cl/447816 mentions this issue: [release-branch.go1.19] cmd/compile: allow ineffectual //go:linkname in -lang=go1.17 and older

[gopherbot]

Closed by merging 9e0b6c1 to release-branch.go1.19.