Ignore pthread_getconcurrency/pthread_setconcurrency.

They're marked obsolescent in POSIX, don't clearly mean anything, aren't
portable because the values don't mean anything, and are no-ops in other
C libraries that do "implement" them.

Bug: N/A
Test: N/A
Change-Id: I07342a0a6a5f6616a8432bfea24ed944c7971d27