RFR (2nd round) 8071627: Code refactoring to override == operator of Symbol*

[Calvin Cheung]

Please review this second version of the fix.

This version has 2 new functions (equals() and not_equals()) in the 
Symbol class.
It replaces the Symbol* == and != comparisons with those 2 function calls.

Pro:

It has a much smaller changeset than the first version.

Con:

Someone may by mistake introduce a new line of (Symbol* == Symbol*).

We will mitigate this by enhancing our internal static analysis tool to 
flag the
above code in the future.

JBS: https://bugs.openjdk.java.net/browse/JDK-8071627

webrev: http://cr.openjdk.java.net/~ccheung/8071627/webrev.01/

Tests:
     JPRT (almost done)
     Will do more perf testing after JPRT

thanks,
Calvin