Turn on LLVM's "basic regalloc" by default

Can still switch back to the default regalloc using the -b flag,
but it looks like the basic register allocator cuts the LLVM JIT
time significantly, and improves every one of our benchmarks.
Refactor interpreter slightly in preparation for resume-at-any-point

Previously things were hardcoded to only allow a single entry point to
the interpreter (since that is how the unwinder determines if the frame
is an interpreter frame).

Instead of having astInterpretFunction be the interpreter frame, make
execute() the frame.  Have to make it a static class function so we can
take its address.
Had to do this for 14.04

I wonder if other people have run into this.
Rename ExcInfo -> UnwindInfo

and exc_info -> unw_info

(This makes more sense with the next change, but I wanted
to separate this large renaming from the functional change.)
Change the way we pass line numbers to the runtime

Before, for compiled functions we used dwarf line number information.

Now it's switched to using the new frame introspection support, and
we pass through the AST_stmt* as one of the arguments.  This gets us
the line number and also the definition of execution point.
wip