RFR: JDK-8140593: Add configure parameter for devkit for the build compiler

[Erik Joelsson]

In Jake, when crosscompiling, we need to build a subset of the jdk, 
including hotspot, to be able to run jmod and jlink. Currently, the 
build compiler is just found on the system so such a configuration 
requires the build system to basically be able to compile the jdk 
without a devkit. The idea with devkits was partly to avoid this. The 
solution would be to add another configure parameter that defines a 
devkit for the build compiler.

Bug: https://bugs.openjdk.java.net/browse/JDK-8140593
Webrev: http://cr.openjdk.java.net/~erikj/8140593/webrev.01/

/Erik