stdenv: avoid -p flag for strip when boostrapping x86_64-darwin

The stdenv wouldn't build with it, as
compiler-rt-libc-11.1.0/lib/darwin/libclang_rt.*_osx.a
retained reference to SDKs (which we forbid for final stdenv).

Assigned authorship to Trofi; I just bisected and added condition.
#224669 (comment)

pkgs/stdenv/darwin/default.nix

@@ -63,6 +63,8 @@ rec {
     unset SDKROOT
 
     stripAllFlags=" " # the Darwin "strip" command doesn't know "-s"
+  '' + lib.optionalString localSystem.isx86_64 ''
+    stripDebugFlags="-S" # the Darwin "strip" command does something odd with "-p"
   '';
 
   bootstrapTools = derivation ({