builder: parallelize most of the build

This commit parallelizes almost everything that can currently be
parallelized. With that, it also introduces a framework for easily
parallelizing other parts of the compiler.

Code for baremetal targets already compiles slightly faster because it
can parallelize the compilation of supporting assembly files. However,
the speedup is especially noticeable when libraries (compiler-rt,
picolibc) also need to be compiled: they will be compiled in parallel
next to the Go files using all available cores. On my dual core laptop
(4 cores if you count hyperthreading) this cuts compilation time roughly
in half when compiling something for a Cortex-M board after running
`tinygo clean`.

Author: aykevl