RFR: 8217442: Optimize native accesses to String.value

[Claes Redestad]

Hi,

operations like jni_GetStringLength, jni_GetStringUTFLength access
String.value multiple times, which emits an access barrier each time.

Streamlining the API so that String.value is only accessed once lead to
a measurable speed-up (including a tiny startup improvement).

Bug:    https://bugs.openjdk.java.net/browse/JDK-8217442
Webrev: http://cr.openjdk.java.net/~redestad/8217442/open.00/

Testing: tier1-3 (ongoing/completed on some platforms)

/Claes