cmd/internal/obj/x86: add missing legacy insts

cmd/internal/obj/x86: add missing legacy insts

Minimizes the amount of "TODO" stuff in test suite
of cmd/asm/internal/asm/testdata/amd64enc.s.

Some instructions were already implemented, but
test cases for them were commented-out.

Does not enable MMX instructions, calls/jumps and some
segment registers instructions.

-- Affected instructions --
BLENDVPD, BLENDVPS
BSWAPW
CBW
CDQE
CLAC
CLFLUSHOPT
CMPXCHG16B
CRC32B, CRC32L, CRC32W
CWDE
FBLD
FBSTP
FCMOVB
FCMOVBE
FCMOVE
FCMOVNB
FCMOVNBE
FCMOVU
FCOMI
FCOMIP
IMUL3L, IMUL3Q, IMUL3W
ICEBP, INT
INVPCID
LARQ
LGDT, LIDT, LLDT
LMSW
LTR
LZCNTL, LZCNTQ, LZCNTW
MONITOR
MOVBELL, MOVBEQQ, MOVBEWW
MOVBQZX
MOVQ
MOVSWW, MOVZWW
MWAIT
NOPL, NOPW
PBLENDVB
PEXTRW
RDPKRU
RDRANDL, RDRANDQ, RDRANDW
RDSEEDL, RDSEEDQ, RDSEEDW
RDTSCP
SAHF
SGDT, SIDT
SLDTL, SLDTQ, SLDTW
SMSWL, SMSWQ, SMSWW
STAC
STRL, STRQ, STRW
SYSENTER, SYSENTER64
SYSEXIT, SYSEXIT64
SHA256RNDS2
TZCNTL, TZCNTQ, TZCNTW
UD1, UD2
WRPKRU
XRSTOR, XRSTOR64
XRSTORS, XRSTORS64
XSAVE, XSAVE64
XSAVEC, XSAVEC64
XSAVEOPT, XSAVEOPT64
XSAVES, XSAVES64
XSETBV

Fixes #6739

Change-Id: I8b125d9a5ea39bb4b9da7e66a63a16f609cef376
Reviewed-on: https://go-review.googlesource.com/97235
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>