Don't run static constructors on arbitrary user exports.

Don't run static constructors on arbitrary user exports.

Previously, "new-style commmands" considered every user-defined
export to be a potential command entrypoint, so wasi-libc and wasm-ld
cooperated to run the user's static constructors on each entrypoint.

This form of new-style command turned out not to be useful, and it
interferes with some use cases, so disable it.

This is done by making an explicit call to `__wasm_call_ctors`, which
tells wasm-ld that it shouldn't synthesize any calls to
`__wasm_call_ctors` on its own.