RFR(S): 8193927: Optimize scanning code for oops.

[Lindenmaier, Goetz]

Hi,

Some platforms don't emit immediate oops to the code. If so, scans 
of the code for oops can be skipped. 

Add flag ImmediateOopsEmitted to each platform specifying the behavior. 
Only search code for immediate oops if this flag is set. Make sure no 
oops are emitted to code if the flag is not set.

@aarch people: should the flag set to 'false' on aarch/arm?  So far it
is true, which defaults to the old behavior.

Please review this change. I please need a sponsor.
http://cr.openjdk.java.net/~goetz/wr17/8193927-oopsInCode/webrev.01/

Best regards,
  Goetz