These directories contain example yasm plugins.
Yasm is only capable of loading plugins when it was built using cmake.
To build yasm with cmake on Unix, from the yasm source tree, do:
  mkdir objdir
  cd objdir
  cmake ..
  make

The plugins are written to be compiled against an *installed* yasm.
Plugins may be loaded on the yasm command line using the -N command line
option, e.g.:
  yasm -N ./libdbgmod.so
  yasm -N Release/dbgmod.dll
  yasm -N /usr/local/lib/libx86mod
(the .so will be automatically appended)
If no directory path is specified, yasm will search in standard library
locations (e.g. LD_LIBRARY_PATH, the rpath of the yasm executable, etc) to
try to load the plugin.  Thus the last example (after installing the plugin)
could likely be written:
  yasm -N x86mod

Plugins may override builtin modules like x86.