RFR: 8203410: Zero: Disable jfr feature by default

[Severin Gehwolf]

Hi,

Currently the jfr (Flight Recorder) feature is being built by default
for Zero JVMs. However, it's not clear whether there will be jfr
support for the Zero variant JVM. At this point, when
StartFlightRecording option is being used for a Zero JVM it
asserts/crashes. It seems more appropriate to disable the feature for a
default build and require one to enable the feature explicitly via:

configure --with-jvm-feature=jfr --with-jvm-variants=zero

We can revisit this change of default once there is good Zero+JFR
support, which currently isn't the case.

Bug: https://bugs.openjdk.java.net/browse/JDK-8203410
webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8203410/webrev.01/

Testing: Verified that the StartFlightRecording option is unrecognized
for a default --with-jvm-variants=zero build.

Thoughts?

Thanks,
Severin