Fix problems compiling with gcc 10

[keithc-ca]

Use a flexible array for RegisterDependencyGroup._dependencies, and promote architecture-specific fields so they don't follow that flexible array.

Fixes: eclipse-openj9/openj9#14219.

[keithc-ca]

Windows is not cooperating:

D:\a\1\s\compiler/codegen/OMRRegisterDependency.hpp(215): warning C4200: nonstandard extension used: zero-sized array in struct/union [D:\a\1\s\build\fvtest\compilertest\testcompiler.vcxproj]
  D:\a\1\s\compiler/codegen/OMRRegisterDependency.hpp(215): note: This member will be ignored by a defaulted constructor or copy/move assignment operator
  omrportTest.cpp
D:\a\1\s\compiler\x\codegen/OMRRegisterDependency.hpp(65): error C2503: 'OMR::RegisterDependencyGroup': base classes cannot contain zero-sized arrays [D:\a\1\s\build\fvtest\compilertest\testcompiler.vcxproj]
  JBCodeCacheManager.cpp

And it seems the added assertion needs to be relaxed:

5: Test command: /home/vsts/work/1/s/build/jitbuilder/release/iterfib
5: Test timeout computed to be: 10000000
5: Assertion failed at /home/vsts/work/1/s/compiler/codegen/OMRRegisterDependency.hpp:67: numDependencies > 0
5: 	Number of dependencies must be positive

[keithc-ca]

Updated to make allowances for MSVC and to relax the new assertion.

[keithc-ca]

jenkins build all

[keithc-ca]

Apparently, it's only GCC that supports extending a class containing a flexible array member: adjusted accordingly.

Longer term we ought to eliminate extensions to OMR::RegisterDependencyGroup so a flexible array member can be used properly with all compilers.

[keithc-ca]

jenkins build all

[keithc-ca]

Updated so clang is not treated as gcc.

[keithc-ca]

jenkins build all

[AdamBrousseau]

Jenkins build xlinux

[keithc-ca]

jenkins build all

[dsouzai]

@jdmpapin do you mind reviewing this one?

[dsouzai]

As mentioned in the slack channel, zos build passing depends on #6292

[keithc-ca]

jenkins build zos

[keithc-ca]

@jdmpapin Do you have any concerns with this?