RFR(s): 8068582: UseSerialGC not always set up properly

[Per Liden]

Hi,

When no Use*GC flag is specified on the command-line we make a default 
GC selection. In case we select SerialGC we fail to set UseSerialGC to 
true (unlike when we select ParallelGC where we set UseParallelGC). This 
means we can run with all Use*GC set to false (which implicitly means 
we're using serial). It also means we can't use UseSerialGC as the sole 
indicator that we're using serial.

This patch makes sure that we always set the corresponding Use*GC for 
the GC we have selected.

A typical case where the VM selects serial but fails to set UseSerialGC 
is when running a client VM without specifying a Use*GC option on the 
command-line.

Testing: Added a new jtreg test, passes jprt

Webrev: http://cr.openjdk.java.net/~pliden/8068582/webrev.0/
Bug: https://bugs.openjdk.java.net/browse/JDK-8068582

/Per