[lld][WebAssembly] Add support for -soname

This change writes the module name to the name section of the wasm
binary.  We use the `-soname` argument to determine the name and we
default the output file basename if this option is not specified.

In the future we will likely want to embed the soname in the dylink
section too, but this the first step in supporting `-soname`.

Differential Revision: https://reviews.llvm.org/D158001