[WebAssembly] Rename eh-assembly.s to -legacy.s

The plan was to make `eh-assembly.s` contain both the legacy and the new
tests, but the new tests require `--no-type-check` because the type
checker for the new EH is in progress. In case this drags on further
than expected, this renames the current file to `-legacy.s` in order to
follow the current naming scheme in `test/CodeGen/WebAssembly`.

After landing this first, `eh-assembly-new.s` in llvm#108668 will be renamed
to `eh-assembly.s`.