Add function names to asm.js tests.

This ensures that they are valid JavaScript fragments, as well as valid
asm.js modules.
Use local copy of acorn for benchmarking.
Fix description of asm.js issues in asmjs.html.
asm-llvm: Use very generous call-site coercion rules.

See https://bugzilla.mozilla.org/show_bug.cgi?id=883175#c4
asm-llvm: operands to binary - should be double, not doublish.

See https://github.com/dherman/asm.js/issues/72

This change was made in spidermonkey in
https://bugzilla.mozilla.org/show_bug.cgi?id=864600#c6

It's not strictly necessary for type safety, since we no longer rely on
use-site coercion to determine types.