lang/bun: Partially respect CPUTYPE
Bun has a very complex build system using Zig and Rust languages.
It is now respecting CPUTYPE for the most part, but it uses the
bundled FreeBSD binary WebKit with bun-related patches which still
brings in some x86-64-v4 and latest SSE instructions.
This can be resolved by creating a dedicated port for bun-patched
WebKit.