RFR [XS]: 8239456: [win][x86] vtable stub generation: assert failure (code size estimate)

[Schmidt, Lutz]

Dear Community,

may I please request reviews for this tiny change.

The vtable stub generator issues a VM call which is highly dynamic in size. The hardwired size estimate has proven to be a bit too small at times. We ran into the related assert failure when using -XX:+DebugVtables. The change simply increases the estimate. 

Bug:    https://bugs.openjdk.java.net/browse/JDK-8239456 
Webrev: http://cr.openjdk.java.net/~lucy/webrevs/8239456.00/ 

Testing: SAP-internal nightly tests (OK), jdk/submit (pending).

Thanks,
Lutz