* Rely on libSystem when targeting macOS.
* Make tools/gen_outline_atomics.zig more idiomatic.
* Remove the CPU detection / auxval checking from compiler_rt. This
functionality belongs in a different component. Zig's compiler_rt
must not rely on constructors. Instead it will export a symbol for
setting the value, and start code can detect and activate it.
* Remove the separate logic for inline assembly when the target does or
does not have lse support. `.inst` works in both cases.
