configure: Fix #21712 again
This is a bit of a shot in the dark to fix #24033 (closed), which appears to be
another instance of #21712 (closed). 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 (closed) but ultimately the
check added for #21712 (closed) 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 (closed).