Implement generic Linux upcalls

[acw]

Many of the upcalls that musl will make into our fake Linux kernel are going to be generic across all the platforms, or have obvious defaults that work for most platforms. We should write all these up in the base HaLVM libraries, so we don't have to worry about them.

For functions that probably won't need to be overwritten for a platform, but may, we can use ld's weak symbol functionality, so that they get overwritten as necessary. Musl is already going to do this, so we might as well jump on the bandwagon.