math/mpx-mul4-*.S: Use more portable type syntax for ambiguous instructions.

math/mpx-mul4-*.S: Use more portable type syntax for ambiguous instructions.

Specifically, replace `adcd MEM, 0' by `adc dword ptr MEM, 0'.  This
removes one reason why Clang's defective assembler won't work, but there
are others.

This is not part of a concerted effort to improve Clang support.
Honestly, as far as I'm concerned, `CCASFLAGS=-fno-integrated-as
-Wno-unicode' is sufficient support for building Catacomb using
Clang.  (That said, I don't actively object to supporting Clang: it's
just not something I want to put much effort into.  I'm happy to accept
tasteful patches which improve Clang support.)  But in retrospect, using
`adcd' here was kind of bletcherous in its own right, and it should be
fixed.