RFR (XS) 8213383 : Wrap up pthread_cond_wait into a loop to workaround potential spurious wakeups

[Ivan Gerasimov]

Hello!

In src/java.base/unix/native/libjsig/jsig.c:

signal_lock() makes all the thread that are not jvm wait while the later 
is installing signal handlers.
This is done via a call to pthread_cond_wait().
Spurious wakeups from pthread_cond_wait() are allowed, so it needs to be 
wrapped up into a loop.

Would you please help review the trivial fix?
Control build and testing of tier1,2,3 went fine on all supported platforms.

BUGURL: https://bugs.openjdk.java.net/browse/JDK-8213383
WEBREV: http://cr.openjdk.java.net/~igerasim/8213383/00/webrev/

Thanks!

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With kind regards,
Ivan Gerasimov