Handle the AR environment variable, used by gccgo,
the same way we handle the CC environment variable.
This ports https://go.dev/cl/526275 to the gofrontend repo.
This is needed for gccgo testing because the top-level GCC Makefile
now passes a --plugin option to ar if it supports one.
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/526295
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