Update target in mozconfig-windows-x86_64 and mozconfig-windows-i686

We need to update x86_64-w64-mingw32 and i686-w64-mingw32 to x86_64-pc-windows-gnu and i686-pc-windows-gnu in mozconfig-windows-x86_64 and mozconfig-windows-i686.

Support for the old targets was dropped in https://hg-edge.mozilla.org/mozilla-central/rev/8b162a9de520.