/usr/share/X11/xkb/keycodes/evdev is not an "evdev file", it's one of the many source files for the X keyboard translation table (xkb). No, it's not loaded upon login. Instead it serves as a source file for the compiler
xkbcomp, and the output of the compiled filed are loaded when the X server starts. The whole topic is a bit complex, so don't go about making changes randomly and expect stuff to work. Also, the usual way to make your own keyboard mapping is to write new files, and leave the old ones in place. See for example An Unreliable Guide to XKB Configuration for an introduction.
If you want to make your right alt key work as control key, a better way is to leave those files as they are, make an
~/.Xmodmap file with the following two lines
remove mod4 = Alt_R
add control = Alt_R
test that it works by looking at
xmodmap -pk for the old state, then loading it with
xmodmap ~/.Xmodmap, then do again an
xmodmap -pk and make sure it looks right.
The result of this will be that the right alt key still shows up under its own keycode, but is treated as the control modifier so you can use it in combination with other keys.
Some display managers load
~/.Xmodmap by default on login, some of the new display managers have lost this feature and you'll have to configure them to do that.