misc: Fix mismatched struct/class "tags" and reenable that warning.

The mismatches were from places where Params structs had been declared
as classes instead of structs, and ruby's MachineID struct.

A comment describing why the warning had been disabled said that it was
because of libstdc++ version 4.8. As far as I can tell, that version is
old enough to be outside the window we support, and so that should no
longer be a problem. It looks like the oldest version of gcc we
support, 5.0, corresponds with approximately libstdc++ version 6.0.21.

https://gcc.gnu.org/onlinedocs/libstdc++/manual/abi.html#abi.versioning

Change-Id: I75ad92f3723a1883bd47e3919c5572a353344047
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40953
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
16 files changed