[Bug target/85100] __builtin_cpu_supports avx does not verify OS supports it

[njs at pobox dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85100

--- Comment #5 from Nathaniel J. Smith <njs at pobox dot com> ---
Julian, are you able to test the patch? I don't have a reproduction setup
currently...