In codegen.zig, the std.Target.Cpu.Arch is now generally available as a
comptime value where needed. This is a tradeoff that causes the compiler
binary to be more bloated, but gives us higher performance, since the
optimizer can optimize per architecture (which is usually how compilers
are designed anyway, with different code per-architecture), and it also
allows us to use per-architecture types, such as a Register enum that is
specific to the comptime-known architecture.
Adds abiSize method to Type.
