Fix the build script to include arm-specific tunnings

[inikulin]

we missing this code branch from the build script: https://github.com/cloudflare/quiche/blob/5b3ab16751e8e0e51d364a49601a0040abbe1b4d/quiche/src/build.rs#L131-L142

[jyn514]

@inikulin why does quiche build boringssl itself instead of going through boring-sys? It seems hard to keep the two in sync, and I'm not sure what purpose it serves.