Implement asm.js style exception handling for Wasm (#664)

* Implement asm.js style exception handling for Wasm

Converts invoke wrapper names generated by LLVM backend to real invoke wrapper
names that are expected by JavaScript glue code.
This is required to support wasm exception handling (asm.js style).

LLVM backend lowers
  invoke @func(arg1, arg2) to label %invoke.cont unwind label %lpad
into
... (some code)
  call @invoke_SIG(func, arg1, arg2)
... (some code)
SIG is a mangled string generated based on the LLVM IR-level function
signature. In LLVM IR, types are not lowered yet, so this mangling scheme
simply takes LLVM's string representtion of parameter types and concatenate
them with '_'. For example, the name of an invoke wrapper for function void
foo(struct mystruct*, int) will be "__invoke_void_%struct.mystruct*_int".  This
function converts the names of invoke wrappers based on their lowered argument
types and a return type. In the example above, the resulting new wrapper name
becomes "invoke_vii".

* Address comments

Change variable names to camelcase
Add a small (semi-)handwritten test case

* Export malloc and free from wasm when available

* Add a test case for exporting malloc/free feature

+ cosmetic 'file' name change in text_before_type.s

* fixInvokeWrapper -> fixEmExceptionInvoke

* Add a TODO

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