[jdk11u-dev] RFR: 8209611: use C++ compiler for hotspot tests

[Goetz Lindenmaier]

I want to downport this for parity with jdk11.0.x-oracle.

I had to resolve /make/common/TestFilesCompilation.gmk
One chunk is pointless, as it was changed again by "8210920: Native C++ tests are not using CXXFLAGS"
which, in head, came after this change, but was already downported to 11u.
I resolved the other chunk that did not apply to the current syntax in head.

I had to conserve some changes we backported from the .cpp files
to the .c files and move them over to the new .cpp files:

test/hotspot/jtreg/vmTestbase/nsk/jvmti/GetClassMethods/getclmthd007/getclmthd007.c
test/hotspot/jtreg/vmTestbase/nsk/jvmti/scenarios/extension/EX03/ex03t001/ex03t001.c
test/hotspot/jtreg/vmTestbase/nsk/jvmti/unit/extmech/extmech.c

In this file, we disable some warnings in 11 with "8232084: HotSpot build failed with GCC 9.2.1"
I can not check whether the warnings are a problem in cpp, too, so
I moved the change to the new file:
test/hotspot/jtreg/vmTestbase/nsk/share/jvmti/jvmti_tools.c

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Commit messages:
 - Backport 8c47dc4a946723de202c943bc577765cefa8d24a

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/634/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=634&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8209611
  Stats: 280942 lines in 2523 files changed: 140728 ins; 140205 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/634.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/634/head:pull/634

PR: https://git.openjdk.java.net/jdk11u-dev/pull/634