x86: drop some stray/bogus DefaultSize

x86: drop some stray/bogus DefaultSize

Insns permitting only GPR operands (and hence implicit sizing when
there's no suffix) don't ever have their DefaultSize attribute
inspected, so it shouldn't be there in the first place.

Additionally XBEGIN is like JMP, not CALL, and hence shouldn't be
converted to 32-bit operand size in .code16gcc mode. While the same is
true for SYSRET, it permitting more than one suffix makes it FLDENV-
like, and hence rather than dropping the attribute, for now add it to
the exclusion list to avoid it getting an operand size prefix emitted
in .code16gcc mode. (This will be dealt with later, perhaps together
with FLDENV and friends.)