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

[Magnus Ihse Bursie]

When reworking the JVM feature handling, I wanted to try to compile 
Hotspot with various features enabled/disabled. I quickly found out that 
it's not really possible to build hotspot without the serial gc. While 
this is not a terribly important use case, I think it's good to be able 
to select serial freely, just as with the other collectors.

With this patch it is possible to build a truly minimal JVM using 
'configure --with-jvm-variants=custom --with-jvm-features=g1gc'.

Bug: https://bugs.openjdk.java.net/browse/JDK-8240224
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8240224-building-without-serial-gc/webrev.01

/Magnus