valgrind: fix build on darwin

We fixed a race condition in #51505 and #51107. This required running
autoreconfHook to pick up the `coregrind-makefile-race.patch` patch.

Unfortunately this broke darwin's postPatch fixes as autoreconfHook would run
afterwards regenerating the fixed makefiles.

Moving the postPatch fixes to preConfigure should resolve the issue.

I left `postPatch = ""` in to avoid a rebuild on linux.