Massive performance loss from OS::sleep hack

[Kris Kennaway]

Kurt Miller wrote:
> David Xu confirmed for me that pthread_yield() does give some
> time to lower priority threads on 7.0 using thr. Attached and inline
> are two patches for the 1.5 port that is how I suggest the issue be
> addressed.
> 
> For 7.0 and up default UseThreadPriorities to true and always
> use pthread_yield(). For < 7.0 default UseThreadPriorities to
> false and conditionally use pthread_yield()/os_sleep(). User's
> can toggle UseThreadPriorities with the command line argument
> -XX:+UseThreadPriorities

Do we know that 6.x requires the old behaviour?  Maybe it can default to 
on there too.  Otherwise this looks good to my eyeball (but the 
DEFAULT_LD_LIBRARY_PATH change looks unrelated)

-#define DEFAULT_LD_LIBRARY_PATH "/usr/lib" /* See ld.so.1(1) */
+#define DEFAULT_LD_LIBRARY_PATH "/usr/lib:/usr/local/lib" /* See 
ld.so.1(1)

Kris