Hopefully fix the sim-eng build, part 2.

This is kinda gross, but I can't find a good way to check for the
existence of gettid(), except by finding its syscall number.  Then
might as well just use it rather than hope gettid's around, as it's
not in sim-eng.

Change-Id: Ieb7b39426dec08bd715b6fe1a9ab5b2801bdf775