RFR: 8273797: Stop impersonating "server" VM in all VM variants

[Aleksey Shipilev]

As the follow-up for Zero-specific JDK-8273494, we might want to clean up build system logic for all VM variants: stop impersonating "server" VMs for all of them. This basically leaves "core" and "custom" variants to be handled with this patch.

Additional testing:
 - [x] Linux x86_64 `core` build passes, `libjvm.so` now in `core`, `jvm.cfg` mentions `-core KNOWN`
 - [x] Linux x86_64 `custom` (+serialgc,+compiler2) build passes, `libjvm.so` now in `custom`, `jvm.cfg` mentions `-custom KNOWN`

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Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/5526/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5526&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273797
  Stats: 41 lines in 5 files changed: 3 ins; 33 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5526.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5526/head:pull/5526

PR: https://git.openjdk.java.net/jdk/pull/5526