RFR: 8216995: Clean up JFR command line processing

[Claes Redestad]

Hi,

JFR setup could be simplified by using macros defined
in globals_extension.hpp rather than use JVMFlag directly.

On top of looking cleaner this avoids JVMFlag::find_flag, which does a
linear scan of all flags. So a tiny startup optimization (detectable
only with special goggles).

Webrev: http://cr.openjdk.java.net/~redestad/8216995/open.00/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8216995

Testing: tier1-2 (ongoing)

Thanks!

/Claes