Removes all remaining usages of the global system instance. After this,
migration can begin to migrate to being constructed and managed entirely
by the various frontends.

Recent changes to the build system that made more warnings be flagged as
errors caused building via clang to break.

Fixes #4795

Makes our error coverage a little more consistent across the board by
applying it to Linux side of things as well. This also makes it more
consistent with the warning settings in other libraries in the project.

This also updates httplib to 0.7.9, as there are several warning
cleanups made that allow us to enable several warnings as errors.

Port citra-emu/citra#5185: "gdbstub: Fix some gdbstub jankiness"

1. Ensure that register information available to gdbstub is most up-to-date.
2. There's no reason to check for current_thread == thread when emitting a trap.
   Doing this results in random hangs whenever a step happens upon a thread switch.