RFR(XXS) 8229185: Shenandoah: CLDG and CodeCahe purge can overlap Safepoints

[Zhengyu Gu]

CLDG::purge() and CodeCache::purge_exception_caches() can overlap 
safepoints, therefore, can be done without STS.

Bug: https://bugs.openjdk.java.net/browse/JDK-8229185
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8229185/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu