[WebAssembly] Use the default private label prefixes.

[WebAssembly] Use the default private label prefixes.

The MC assembler doesn't like using the empty string as a private label
prefix because then it treats all labels as private. This commit reverts
back to the default prefix, which is .L, which is common in ELF targets
and consistent with the LLVM name mangler.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257083 91177308-0d34-0410-b5e6-96231b3b80d8