This gets gdbstub working at least to the point where clients can
communicate with it.

What works:
- Reading/writing GPRegs
- Reading/writing memory
- Interrupting the emulated program and continuing

What does NOT work:
- Breakpoints. Sizes have been updated to u64, but support will need to be
  added in the interpreter for them to work.
- VRegs. Mostly because my gdb was having issues with 128-bit regs for
  some reason. However, the current u128 representation is a bit
  awkward to use and should probably be updated first.

* Fixes some cast warnings, partially fixes citra #3064

* Converted casts to uint32_t to u32

* Ran clang-format

Core::System().GetInstance().IsPoweredOn() -> Core::System::GetInstance().IsPoweredOn()

Corrects a few issues with regards to Doxygen documentation, for example:

- Incorrect parameter referencing.
- Missing @param tags.
- Typos in @param tags.

and a few minor other issues.

Also uses size_t as the length indicator type, as is common with buffers.

Currently, this is only ever queried, so adding a function to check if the
server is enabled is more sensible.

If directly modifying this externally is ever desirable, it should be done
by adding a function to the interface, rather than exposing implementation
details directly.

This fixes the comments left on the PR (whitespace, SO_REUSEADDR,
comment changes).

Previously, the padding wasn't correctly accounted for which caused the gdbstub to read and write everything after R15 (starting with the dummy FPA registers) incorrectly, which caused CPSR to not be handled correctly. Everything appears to be working as expected with this change.