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Adds -lm linker flag for FreeBSD/OpenBSD and uses github.com/bazelbuil… (#1032)

* Adds -lm linker flag for (Free|Open)BSD and uses github.com/bazelbuild/platforms for platform detection.

* Prefer selects.with_or to select the linkopts.

* @platforms appears to be implicitly available. @bazel_skylib would require updating every dependent repository.

* Re-enable platforms package.