No longer lock WeakGlobalsLock in FindDexCache

Since DecodeWeakGlobal is now usually lock free, it is faster to use
that instead of the locked version.

10x boot.oat compile on host -j4:
Before:
real  1m24.674s
user  3m40.446s
sys 0m15.749s

After:
real  1m14.529s
user  3m26.734s
sys 0m8.469s

Change-Id: I15bb10ad3a9bb2e6f8edb5dc0668267be821d603