testsuite: Add tail_call effective target

The RTL "expand" dump is the first RTL dump, and it also appears to be
the earliest trace of the target having implemented sibcalls.
Including the "," in the pattern searched for, to try and avoid
possible false matches, but there doesn't appear to be any identifiers
or target names nearby so this is just belts and suspenders.  Using
"tail_call" as a shorter and more commonly used term than a derivative
of "sibling calls", and expecting only gcc folks to have heard of
"sibcalls".

	* lib/target-supports.exp (check_effective_target_tail_call): New.