This is a bit of a shot in the dark to fix #24033, which appears to be
another instance of #21712. For some reason the ld-override logic
*still* appears to be active on Darwin targets (or at least one).
Consequently, on misconfigured systems we may choose a non-`ld64`
linker.

It's a bit unclear exactly what happened in #24033 but ultimately the
check added for #21712 was not quite right, checking for the
`ghc_host_os` (the value of which depends upon the bootstrap compiler)
instead of the target platform. Fix this.

Fixes #24033.

(cherry picked from commit f6b2751f)