RFR(S): 8063112: Compiler diagnostic commands should have locking instead of safepoint

[Nils Eliasson]

Hi,

Please review this small change.

The first implementation of compiler diagnostic commands was asserting 
on safepoint or lock. The safepoint was always taken since the dcmd by 
default block for a safepoint. That is not correct, the lock must be 
taken, and a whether we are at a safepoint doesn't matter.

Tested with jtreg tests in Hotspot and JDK that exercise this code.

Bug: https://bugs.openjdk.java.net/browse/JDK-8063112
Webrev: http://cr.openjdk.java.net/~neliasso/8063112/webrev.01/

Thanks,
Nils