x86: adjust monitor/mwait templates

x86: adjust monitor/mwait templates

Architecturally, MONITOR's and MONITORX'es memory operand is a 16- or
32-bit register outside of 64-bit mode, and a 64- or 32-bit register
inside 64-bit mode. The other register operands, including all of them
for MWAIT and MWAITX, are uniformly 32-bit, irrespective of mode. Retain
the original 64-bit MONITOR{,X} templates for compatibility only, and
fold the MWAIT{,X} ones.