[WebAssembly] Teach lld how to demangle "__main_argc_argv".

WebAssembly requires that caller and callee signatures match, so it
can't do the usual trick of passing more arguments to main than it
expects. Instead WebAssembly will mangle "main" with argc/argv
parameters as "__main_argc_argv". This patch teaches lld how to
demangle it.

This patch is part of https://reviews.llvm.org/D70700.