thread_pthread.c: reset timeslice delay when uncontended

This matches the behavior of old timer thread more closely
and seems to fix [Bug #14999] when limited to a single CPU.
I cannot reproduce the error on a multi-core system unless
I use schedtool to force affinity to a single CPU:

schedtool -a 0x01 -e make test-spec \
	MSPECOPT='-R1000 spec/ruby/library/conditionvariable/wait_spec.rb'

While it may be good enough to pass the spec, I don't have
huge degree of confidence in the interrupt handling robustness
under extremely heavy load (these may be ancient bugs, though).

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64467 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread_pthread.h

@@ -53,6 +53,7 @@ typedef struct rb_global_vm_lock_struct {
     /* slow path */
     struct list_head waitq;
     const struct rb_thread_struct *timer;
+    int timer_err;
 
     /* yield */
     rb_nativethread_cond_t switch_cond;