[FatLTO] Detect LLD linker more reliably (#128285)
It's possible to have an `ld-path` point to a linker that doesn't have
the `ld.lld` filename (e.g. linker wrapper that may emit telemetry
before invoking the linker). This was causing mis-compilations with
fatLTO since the check couldn't reliably detect that it was using lld.
Instead, rely on the value from `-fuse-ld` to determine whether lld is
enabled.
