asmc: code style modernization and minor cleanups

• Allow ASMC_DEBUG to be treated as a variadic macro.
• Add ellipses around sizeof(..) calls.
• Reindent some of the lines to better follow style(9) with line wrapping.

A number of other clang-format changes were not taken as they are not
compatible with style(9).

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D55394