Handle VMTYPE for "core" VM variant

Building with --with-jvm-variants=core currently produces a binary that replies an odd version:

$ build/linux-x86_64-core-fastdebug/images/jdk/bin/java -version
openjdk version "18-internal" 2022-03-15
OpenJDK Runtime Environment (fastdebug build 18-internal+0-adhoc.shade.jdk)
OpenJDK 64-Bit VM (fastdebug build 18-internal+0-adhoc.shade.jdk, interpreted mode)

It should actually say "OpenJDK 64-Bit Core VM", I think. Build just misses a simple definition of VMTYPE for core variant.