cmd/link/internal/ld: drop Go 1.12 compatibility on darwin
Go 1.17 is the minimum required version for bootstrap as of Go 1.20.
Also, the package no longer builds with Go 1.12 used for bootstrap.
Thus, drop the Go 1.12 compatibility and used linkname unconditionally.
For #44505
Change-Id: Ic160aba4a33d580987b4633a3db03a025804d090
Reviewed-on: https://go-review.googlesource.com/c/go/+/430335
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