[vector] Make generation of perf tests optional

[Paul Sandoz]

Hi,

Please review this modification to scripts, making the generation of perf tests optional:

  http://cr.openjdk.java.net/~psandoz/panama/vector-gen-test-perf-optional/webrev/ <http://cr.openjdk.java.net/~psandoz/panama/vector-gen-test-perf-optional/webrev/>

This should make it easier to disable the generation of performance tests, and simplify merges from vectorIntrinsics when the perf templates and perf test project are removed from the vector-unstable branch.

Paul.