This is a simplification of the cmake build script which introduces a
new "stage3" target that is built by default, which builds and installs
a stage3 zig.
It greatly simplifies the build instructions for Zig, making it conform
to the regular cmake routine, while still producing a stage3 artifact.
Rather than `zig0 build ...` the build now does
`zig0 build-lib ...`, avoiding the requirement of linking the build
script, and thus avoiding the requirement of finding native libc,
for systems where libc is the system ABI.
