RFR: Fixing GCC 7.3.1 build issues

[Per Liden]

We noticed that building the ZGC repo with a recent gcc (like 7.3.1) 
didn't work (it's more picky and generates additional warnings, etc). So 
here are patches to fix that.

ZGC: Avoid treating oops as volatile in root barriers
http://cr.openjdk.java.net/~pliden/zgc/make_root_barriers_non_volatile/webrev.0

ZGC: Avoid compiler warning about implicit dereference of volatile oop
http://cr.openjdk.java.net/~pliden/zgc/avoid_implicit_dereference_of_volatile_oop/webrev.0

ZGC: Avoid oop cast issue in security manager
http://cr.openjdk.java.net/~pliden/zgc/avoid_oop_cast_issue_in_security_manager/webrev.0

ZGC: TEMPORARY: Avoid compiler warnings about using the deprecated readdir_r
http://cr.openjdk.java.net/~pliden/zgc/temporary_fix_for_deprecated_readdir/webrev.0

I'm also planning on bringing in Erik's fix of JDK-8199685 (Access 
arraycopy build failure with GCC 7.3.1), when that goes in upstream.

/Per