RFR (S) 8228672: [TESTBUG] gc/metaspace/TestSizeTransitions.java fails on 32-bit platforms

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8228672

Fix:
  http://cr.openjdk.java.net/~shade/8228672/webrev.02/

Instead of duplicating @run sections even more, decided to test for Platform.is64bit, like other
tests in vicinity do.

Testing: affected test {x86_64, x86_32}, jdk-submit (running)

-- 
Thanks,
-Aleksey