cmd/compile: allow ineffectual //go:linkname in -lang=go1.17 and older
Prior to Go 1.18, ineffectual //go:linkname directives (i.e.,
directives referring to an undeclared name, or to a declared type or
constant) were treated as noops. In Go 1.18, we changed this into a
compiler error to mitigate accidental misuse.
However, the x/sys repo contained ineffectual //go:linkname directives
up until go.dev/cl/274573, which has caused a lot of user confusion.
It seems a bit late to worry about now, but to at least prevent
further user pain, this CL changes the error message to only apply to
modules using "go 1.18" or newer. (The x/sys repo declared "go 1.12"
at the time go.dev/cl/274573 was submitted.)