Update pvcalarm test

Use pthread_lib_init() instead of uthread_lib_init(), which requires us
to set pthread_can_adjust_vcores to false.  Also, need to initalize
counts to 0, otherwise we miscount and break the timing loop early.
Handle pending signals even when resuming current_uthread
Add SIGPROF based pvcalarms to pthreads

Also added a utest to test this stuff.