module/zstd: pass -U__BMI__

module/zstd: pass -U__BMI__

If kernel is compiled with -march=znver1 or -march=znver2 zstd module
compilation will fail due to SSE register return with SSE disabled.
What's interesting, is that -march=skylake also implies -mbmi which
defines __BMI__ but compilation succeeds.  It is probably due to
different BMI implementations on AMD and INTEL processors and the
way compiler uses instructions.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Georgy Yakovlev <gyakovlev@gentoo.org>
Closes #10758
Closes #10829