Update Windows toolchain for Firefox 78.9 (maint-10.0-desktop)

We will need to update the Windows toolchain to build Firefox 78.9.

MinGW update:

• https://hg.mozilla.org/releases/mozilla-esr78/rev/1b0e1f0d885e

clang update:

• https://hg.mozilla.org/releases/mozilla-esr78/rev/cc840a008393
• https://hg.mozilla.org/releases/mozilla-esr78/rev/cf1ff3a632bf