When linking a WebAssembly binary using wasm-ld, we will now correctly pass
--shared when building a dynamic library and --pie when `-fpic` is given.
This is a breaking change and users who currently build dynamic libraries
for WebAssembly but wish to have an executable without a main, should use
build-exe instead while supplying `-fno-entry`.

We also verify the user does not supply --export-memory when -dynamic is
enabled. By default we set this flag now to 'false' when `-dynamic` is used
to ensure we do not enable it as it's enabled by default for regular
WebAssembly binaries.