[Bug target/84413] -march=skylake-avx512 disables BT instruction

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84413

--- Comment #1 from Uroš Bizjak <ubizjak at gmail dot com> ---
Besides X86_TUNE_USE_BT, there is probably a long list of flags that have to be
enabled for m_SKYLAKE_AVX512 (and m_CANNONLAKE and m_ICELAKE).

Somebody will have to go through all tune flags for the above mentioned
targets.