Why can't RVM find compiler and 'make' tool after installing XCode?
It looks like you have installed Xcode, but not the command line tools that RVM requires to build Ruby and other components (make, gcc, etc)
Here are some instructions (taken from this answer):
Xcode includes a new "Downloads" preference pane to install optional components such as command line tools, and previous iOS Simulators. To open this pane click the "Xcode" button in the top left of the screen near the Apple logo, then click "Preferences", then click "Downloads".
Then install the 'Command Line Tools' bundle by clicking the Install button to the right of it.