Modifies union construction in the debug directory so output is more
amenable to alternative compilers. Verified that this change produces
code that builds with clang, gcc, msvc, nvhpc, aocc, icc, openxl, and
cray hpc.
These were the kinds of errors seen in MSVC, which this patch fixes.
```
debug/Decoder.hh(24): error C2461: 'gem5::debug::unions::Decoder': constructor syntax missing formal parameters
debug/Decoder.hh(31): error C7624: Type name 'gem5::debug::unions::Decoder' cannot appear on the right side of a class member access expression
```
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