RFR: JDK-8240224 Allow building hotspot without the serial gc

[Kim Barrett]

> On Mar 10, 2020, at 7:22 PM, David Holmes <david.holmes at oracle.com> wrote:
> Also I'm not at all clear what happens if the only GC configured is one of the experimental GCs for which we would normally have to set -XX:+UnlockExperimentalVMOptions ??

Yes, it seems wrong to ever select an experimental GC by default.