RFR: 8221723: Avoid storing zero to String.hash

[Claes Redestad]

Hi,

when a String has a calculated hash code value of 0, we recalculate and
store a 0 to the String.hash field every time (except for the empty
String, which is special cased). To make String objects more amenable to
storage in shared read-only memory, e.g., CDS archives, we should avoid
this redundant store.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8221723
Webrev: http://cr.openjdk.java.net/~redestad/8221723/

Testing: tier1-3, no regression on existing and new StringHashCode
micros

/Claes