RFR 8202683: Minimal VM should build cleanly on 64-bit platforms

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8202683

Fix:
  http://cr.openjdk.java.net/~shade/8202683/webrev.01/

Minimal VM is targeted to 32-bit only, but hear me out. Recent build system changes, notably
conditional GC compilation, requires build/testing with some GCs disabled. Ultimately, we want
minimal VM to be buildable at all times. Today, we need to cross-compile to x86_32 to verify that,
but with this change, we can build minimal on x86_64, thus simplifying our day-to-day jobs. The
change itself generates jvm.cfg for minimal VM properly on both bitnesses.

Testing: x86_64 server/minimal builds, x86_32 server/minimal builds

Thanks,
-Aleksey