[WebAssembly] Add default -allow-undefined-file to linker args

Also, don't use the outdated lib32/lib64 naming of files
within the sysroot. The more modern/flexible approach
IIUC is to use seperate sysroots or /lib/<target-tripple>
and /include/<target-tripple>.

Differential Revision: https://reviews.llvm.org/D33565

llvm-svn: 306074