JDK 10 RFR(S): 8173465: Introduce NearLabel for branches known to be short.

[Lindenmaier, Goetz]

Hi

please review this small optimization of Labels.
This so far targets s390, but could be used on other platforms as well.
I please need a sponsor.
http://cr.openjdk.java.net/~goetz/wr17/8173465-NearLbl/webrev.01/index.html

Some branches issued to assembly code are obviously of short distance. Due to separation into shared and platform code this not always is known when the branch instruction is selected. 
NearLabels indicate that a branch with short reach can be used.

Best regards,
  Goetz