Core: Fix warnings in gpu.cpp

Small, general code cleanup 

Digital user input and HID module implementation for PAD

loader.cpp: improved file extension checking, made Upper/LowerStr useful, moved string_util into Common namespace

Prune redundant includes

Removed fixed_size_queue.h

Common: Fix a potential infinite loop in StringUtil's ReplaceAll

Common: Remove HAVE_CXX11_SYNTAX define from Common.h

It's unused and doesn't look like it compiles anyway :/

Instead of forcibly taking the last 4 characters, it now finds the last extension separator (the period) and takes a substr of its location.

core: Make the ARM disassembler use std::string internally

Removed redundant headers for std implementations

core: Pass string by reference in FetchFromPortName and DeleteService

Minor cleanup in ncch.

In C++ you can simply just declare it as is.

Removed common/atomic, instead using std::atomic

Removed the need for X11 on OS X

It only causes issues when someone (who doesn't need it) doesn't have it.

CMake cleanup