AVX-512: Add {evex} instruction prefix

AVX-512: Add {evex} instruction prefix

For instructions that can be encoded either in VEX or EVEX,
{evex} forces nasm to encode in EVEX.

Signed-off-by: Jin Kyu Song <jin.kyu.song@intel.com>

• [DBH] assemble.c
• [DBH] nasm.h
• [DBH] parser.c
• [DBH] tokens.dat