RFR(s): 8150026: Add the ability to log with variable log level

[Robbin Ehn]

Hi, please review.

This adds a write method to the UL log which takes the level as parameter.

JBS: https://bugs.openjdk.java.net/browse/JDK-8150026
Webrev: http://cr.openjdk.java.net/~mlarsson/rehn/8150026/

Tested ok with rbt hotspot_all.

Thanks!

/Robbin