rustc: Re-jigger -L and -l for MSYS compatibility

As discovered in #20376, the MSYS shell will silently rewrite arguemnts that
look like unix paths into their windows path counterparts for compatibility, but
the recently added `:kind` syntax added to the `-L` flag does not allow for this
form of rewriting. This means that the syntax can be difficult to use at an MSYS
prompt, as well as causing tests to fail when run manuall right now.

This commit takes the other option presented in the original issue to prefix the
path with `kind=` instead of suffixing it with `:kind`. For consistence, the
`-l` flag is also now migrating to `kind=name`.

This is a breaking change due to the *removal* of behavior with `-L`. All code
using `:kind` should now pass `kind=` for `-L` arguments. This is not currently,
but will become, a breaking change for `-l` flags. The old `name:kind` syntax is
still accepted, but all code should update to `kind=name`.

[breaking-change]
Closes #20376