Compile mutually-recursing loops into a single asmjs function.

Rather than maintining a single compiled function for each loop, we
now merge together all loops that can potentially jump to one-another
into a single function.  This removes the need for the execution helper
trampoline and should reduce the overhead of calling into jitted asmjs
code.
Do some very basic relooping of the jitted code.
Tweak compile settings to work better with emscripten-fastcomp.
Tweak emscripten build options
Fix infinite loop when merging an AsmJSFunc to itself
Add C declarations of jit helper functions.