bintools-wrapper, cc-wrapper: parameterize darwin min version variable

These variables are the ones that the standard toolchain uses, so we
should use those and not always use MACOSX_DEPLOYMENT_TARGET.

See https://github.com/tpoechtrager/cctools-port/blob/236a426c1205a3bfcf0dbb2e2faf2296f0a100e5/cctools/ld64/src/ld/PlatformSupport.cpp#L54-L55

lib/systems/default.nix

@@ -120,6 +120,10 @@ rec {
       # platforms define the old name "sdkVer".
       darwinSdkVersion = final.sdkVer or "10.12";
       darwinMinVersion = final.darwinSdkVersion;
+      darwinMinVersionVariable =
+        if final.isMacOS then "MACOSX_DEPLOYMENT_TARGET"
+        else if final.isiOS then "IPHONEOS_DEPLOYMENT_TARGET"
+        else null;
 
       emulator = pkgs: let
         qemu-user = pkgs.qemu.override {