[WebAssembly] New-style command support

This adds support for new-style command support. In this mode, all exports
are considered command entrypoints, and the linker inserts calls to
`__wasm_call_ctors` and `__wasm_call_dtors` for all such entrypoints.

This enables support for:

 - Command entrypoints taking arguments other than strings and return values
   other than `int`.
 - Multicall executables without requiring on the use of string-based
   command-line arguments.

This new behavior is disabled when the input has an explicit call to
`__wasm_call_ctors`, indicating code not expecting new-style command
support.

This change does mean that wasm-ld no longer supports DCE-ing the
`__wasm_call_ctors` function when there are no calls to it. If there are no
calls to it, and there are ctors present, we assume it's wasm-ld's job to
insert the calls. This seems ok though, because if there are ctors present,
the program is expecting them to be called. This change affects the
init-fini-gc.ll test.