Add sysroot to the linker flags
Add sysroot to the linker flags

Some tests directly use the `Link` structure to compile, not just link.

Also, the wasi-sdk crate is designed to only compile to wasm, so don't
bother about other targets.

Also remove `AllowUndefined()` that doesn't seem to be supported by
`wasm-ld`.

fastly/lucet,fastly/lucet,fastly/lucet,fastly/lucet,fastly/lucet