Build fails when excluding Serial GC

[Yang Yi]

Hi,

It's quite easy to reproduce this problem:
./configure --with-jvm-features=-serialgc ... ; make images

I got the following output
```
...
=== Output from failing command(s) repeated here ===
* For target hotspot_variant-server_libjvm_objs_genCollectedHeap.o:
/home/qingfeng.yy/openjdk16_so_warning/jdk/src/hotspot/share/gc/shared/genCollectedHeap.cpp: In member function 'virtual void GenCollectedHeap::post_initialize()':
/home/qingfeng.yy/openjdk16_so_warning/jdk/src/hotspot/share/gc/shared/genCollectedHeap.cpp:206:3: error: 'MarkSweep' has not been declared
  206 |   MarkSweep::initialize();
      |   ^~~~~~~~~
* All command lines available in /home/qingfeng.yy/openjdk16_so_warning/jdk/build/linux-x86_64-server-release/make-support/failure-logs.
=== End of repeated output ===
```
I found current JVM features contain the serial gc, but actually I can not 
build an image that does not contain serial gc. This problem has existed 
from jdk 11 to jdk head. I am somewhat surprised, so I haven't filed an
issue on JBS. Is this really a bug? Or actually we should revise the building 
document and remove all INCLUDE_SERIALGC macros?

Cheers,Yang Yi