Porting adopting RI java.lang.Thread/ThreadGroup w/ --enable-openjdk-thread-support to other Java levels

[JasonFengJ9]

Adopting RI java.lang.Thread/ThreadGroup can be enabled via --enable-openjdk-thread-support at JavaNext extension repo. This is to support project loom.

This will be ported to other Java levels according to priorities.

[keithc-ca]

It should be ported to other versions that support --enable-openjdk-thread-support, even if the default is off.

[JasonFengJ9]

Changed to: This will be ported to other Java levels according to priorities.