AppVeyor build optimizations

[zeux]

• Use /MP for cl.exe builds to try to take advantage of 2 cores
• Use -j2 for MinGW builds to do the same
• Don't build AppVeyor on non-master branches to avoid double builds for PRs from this repository

MP and j2 aren't as impactful as I'd like them to be but they do result in ~25% build time reduction. The branch restriction should additionally cut the time for internal PRs 2x.