configure: Fix #21712 again

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.

m4/find_ld.m4

@@ -70,19 +70,23 @@ AC_DEFUN([FIND_LD],[
         AC_CHECK_TARGET_TOOL([LD], [ld])
     }
 
-    if test "$ghc_host_os" = "darwin" ; then
+    case "$target" in
+    *-darwin)
         dnl N.B. Don't even try to find a more efficient linker on Darwin where
         dnl broken setups (e.g. unholy mixtures of Homebrew and the native
         dnl toolchain) are far too easy to come across.
         dnl
         dnl See #21712.
         AC_CHECK_TARGET_TOOL([LD], [ld])
-    elif test "x$enable_ld_override" = "xyes"; then
-        find_ld
-    else
-        AC_CHECK_TARGET_TOOL([LD], [ld])
-    fi
-
+        ;;
+    *)
+        if test "x$enable_ld_override" = "xyes"; then
+            find_ld
+        else
+            AC_CHECK_TARGET_TOOL([LD], [ld])
+        fi
+        ;;
+    esac
     CHECK_LD_COPY_BUG([$1])
 ])