MPX: Adapt GAS's mib syntax with an index reg only

MPX: Adapt GAS's mib syntax with an index reg only

GAS uses *1 multiplier for explicitly marking an index register in mib operand.
e.g.) [rdx * 1 + 3] is equivalent to [3, rdx] in NASM's split EA format
So only for mib operands, this is encoded same as gas does.

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

• [DBH] assemble.c
• [DBH] test/mpx-64.asm