Actually allow halide_library() to use supported_platforms_regex
Actually allow halide_library() to use supported_platforms_regex

Summary:
I noticed that both halide build rules were being executed.
This fix truly implements the regex (at least, well enough for my
needs).

Test Plan:
Get a little farther building a Halide-dependent app on
Linux.

Reviewed By: yiding

fbshipit-source-id: 424f6b1

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