Migrate J9 specific Linkages to appropriate J9 namespace

[0xdaryl]

Includes JNI, Helper, CHelper, and Private linkages specific to OpenJ9.

[0xdaryl]

Jenkins test sanity all jdk11

[0xdaryl]

@ymanton @dsouzai @fjeremic : I would appreciate a review and merge from at least one of you please (when a full set of CI builds can be run).

This is just some cleanup work to move the various J9 linkages into the appropriate namespace. The changes should be fairly straightforward.

[DanHeidinga]

@0xdaryl Is there a design that describes the new(?) namespace approach? It would be good to document what's changing here and why.

[0xdaryl]

Namespaces in the compiler were introduced when the language-agnostic compiler technology (i.e., OMR) was separated from the Java JIT (i.e., OpenJ9). They are used not only to provide project-level separation of code, but for the correct operation of the extensible class mechanism employed by the OMR compiler and Java JIT.

There isn't a lot of documentation on the use of namespaces in the compiler, but there is some provided within OMR:

https://github.com/eclipse/omr/tree/master/compiler#namespaces

Further info on extensible classes and its use of namespaces is here:

https://github.com/eclipse/omr/blob/master/doc/compiler/extensible_classes/Extensible_Classes.md#naming-and-namespaces

When OMR and OpenJ9 were open sourced, there was a lot of refactoring work still to be done that was left as an exercise to be completed over time as resources permitted. One of those pieces of work was to continue to move code into the appropriate namespaces in both the OMR and OpenJ9 projects. The work in this PR does that for one single class hierarchy (the Linkage hierarchy) in OpenJ9. It makes the class names and file names consistent across all architectures.

Removing the TR_ smurfing and making proper use of the TR:: namespace is also an ongoing goal (eclipse/omr#3519).

[fjeremic]

Our x86 infrastructure is still offline. I'd prefer to wait until Monday until those issues are resolved so we can re-launch those builds and ensure no breakage if that is ok.

[0xdaryl]

Sure, that's no problem. Thanks.

[fjeremic]

Jenkins test sanity all jdk11

[0xdaryl]

Jenkins test sanity all jdk11

[knn-k]

I am seeing a build break with AArch64, which seems to be related to this PR:

/root/openj9-openjdk-jdk11/build/linux-aarch64-normal-server-release/vm/compiler/../compiler/aarch64/codegen/ARM64PrivateLinkage.cpp:47:69: error: ISO C++ forbids declaration of 'ARM64PrivateLinkage' with no type [-fpermissive]
 J9::ARM64::PrivateLinkage::ARM64PrivateLinkage(TR::CodeGenerator *cg)
                                                                     ^
/root/openj9-openjdk-jdk11/build/linux-aarch64-normal-server-release/vm/compiler/../compiler/aarch64/codegen/ARM64PrivateLinkage.cpp:47:69: error: no 'int J9::ARM64::PrivateLinkage::ARM64PrivateLinkage(TR::CodeGenerator*)' member function declared in class 'J9::ARM64::PrivateLinkage'
make[6]: *** [/root/openj9-openjdk-jdk11/build/linux-aarch64-normal-server-release/vm/compiler/../objs/compiler/aarch64/codegen/ARM64PrivateLinkage.o] Error 1

See #7837.