lld accepts both syntaxes, but we were rejecting (and, before
3f7e9ff597a3514bb1c4f1900027c40682ac9f13, ignoring) the former.

In particular, "cargo-zigbuild" was broken since Rust
unconditionally adds "-znoexecstack" (not "-z noexecstack")
on non-Windows platforms.

Co-authored-by: Andrew Kelley <andrew@ziglang.org>