For linking static libraries, rustc will use whatever `cc` it finds (or
the equivalent on Windows).  For our purposes, however, `cc` is not what
we use to link and we may have additional options we would like to pass
to the linker.

To do this, we need to tell Cargo about our alternate linker (currently
only used for target compilations, on the theory that the host compiler
rustc finds is probably good enough) and we also need to pass our linker
options into the process.  We do this with environment variables, which
is not a great solution, but works surprisingly well.

This alternate linker is disabled for ASan builds due to peculiar
crashes when running Rust build scripts and for Windows, because we
don't do any interesting cross-compiling there.