K6-2+ and 6x86MX missing instructions

[Torinde]

K6-2+, K6-III+ and 6x86MX are listed cpu.h and readme, but I don't see the extra instructions added by those CPUs in the code

K6 : SYSEXIT
K6-2+ : PF2IW, PFNACC, PFPNACC, PI2FW, PSWAPD
6x86MX : PAVEB, PADDSIW, PMAGW, PDISTIB, PSUBSIW, PMVZB, PMVNZB, PMVLZB, PMVGEZB, PMULHRIW, PMACHRIW; FTSTP, FRINT2, FRICHOP, FRINEAR

Of course, I couldn't find also the 3DNow! Pro from Geode GX/LX) : PFRSQRTV, PFRCPV - but those CPUs aren't listed, so that's as expected.

Related to joncampbell123/dosbox-x#3217