linux: add an option for clang

It will be used on the future with clang/cmake cleaning contribution
pcsx2/common: fix tons of warning spamming on clang

* Don't use parameter name to avoid unused parameter
* Set a default for various case
Add a CMake compile time option for building a 64bit binary.

By default the cmake build will still cross compile a 32bit binary and spout a message about it if not enabled.
This doesn't fix the 64bit build issues, just makes it easier for someone to test 64bit builds in the future.
Look towards a bright future instead of a dark and gloomy past
cmake: clean main file and option

Move build configuration from CMakeLists to cmake/BuildParameters.cmake where it belongs
Use option syntax for on/off option
Complete a bit previous commit to force all 64bits path detections (likely done by cmake)
cmake: reduce a bit cmake syntax cluttering
Merge branch 'Sonicadvance1-cmake_64bit_option'
Merge pull request #1 from PCSX2/master

Am i doing this correctly?