RFR(xs): JDK-8152329: jvm should treat the "Multi-Release" jar manifest attribute name as case insensitive

[Calvin Cheung]

Please review this small fix for:
https://bugs.openjdk.java.net/browse/JDK-8152329

The fix is to convert the buffer containing the jar manifest into lower 
case before performing strstr.

webrev: http://cr.openjdk.java.net/~ccheung/8152329/webrev.01/

Testing:
     JPRT -testset hotspot
     The tests under runtime on the platforms built by JPRT.

thanks,
Calvin