[WebAssembly] Make sign-extension opcodes a distinct feature.

Sign-extension opcodes have been split into a separate proposal from
the main threads proposal, so switch them to their own target
feature. See:

https://github.com/WebAssembly/sign-extension-ops


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@322966 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed