RFR: Update thread local bad mask during root scanning

[Per Liden]

Patch to update the thread local address bad mask during root scanning 
while we're anyway walking the thread list (in parallel), instead of 
doing a separate single threaded walk over that list.

This patch also removes some platform dependent code, and updates the 
thread local bad mask on all platforms, even if it's strictly not needed 
on e.g. Solaris/SPARC. However, it doesn't hurt performance wise, and 
keeps the model nicely homogeneous across platforms.

Webrev: 
http://cr.openjdk.java.net/~pliden/zgc/update_thread_local_address_bad_mask/webrev.0

cheers,
Per