RFR(S/M): 8150646: Add support for blocking compiles through whitebox API

[Nils Eliasson]

Hi,

Please review this change that adds support for blocking compiles in the 
whitebox API. This enables simpler less time consuming tests.

Motivation:
* -XX:-BackgroundCompilation is a global flag and can be time consuming
* Blocking compiles removes the need for waiting on the compile queue to 
complete
* Compiles put in the queue may be evicted if the queue grows to big - 
causing indeterminism in the test
* Less VM-flags allows for more tests in the same VM

Testing:
Posting a separate RFR for test fix that uses this change. They will be 
pushed at the same time.

RFE: https://bugs.openjdk.java.net/browse/JDK-8150646
JDK rev: http://cr.openjdk.java.net/~neliasso/8150646/webrev_jdk.01/
Hotspot rev: http://cr.openjdk.java.net/~neliasso/8150646/webrev.02/

Best regards,
Nils Eliasson