Fix building sha512-avx512 with clang
* cipher/sha512-avx512-amd64.S (_gcry_sha512_transform_amd64_avx512): Change "%xmm??" registers to "xmm??" for clear_reg parameter. * configure.ac (gcry_cv_gcc_inline_asm_avx512): Check support for registers in range "zmm16-zmm31".
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>