Make BMI2 inline assembly check more robust
* configure.ac (gcry_cv_gcc_inline_asm_bmi2): New assembly test.
Use actual assembly snippets from keccak.c to check that compiler
has proper support for used BMI2 instructions.
- GnuPG-bug-id: T3408
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>