Currently, rustc defaults to invoking `cc`, even if `HOSTCC` is defined,
resulting in build failures in hermetic environments where `cc` does not
exist. This includes both hostprogs and proc-macros.
Since we are setting the linker to `HOSTCC`, we set the linker flavor to
`gcc` explicitly.
Signed-off-by: Matthew Maurer <mmaurer@xxxxxxxxxx>
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