* use the real start code for LLVM backend with x86_64-linux
   - there is still a check for zig_backend after initializing the TLS
     area to skip some stuff.
 * introduce new AIR instructions and implement them for the LLVM
   backend. They are the same as `call` except with a modifier.
   - call_always_tail
   - call_never_tail
   - call_never_inline
 * LLVM backend calls hasRuntimeBitsIgnoringComptime in more places to
   avoid unnecessarily depending on comptimeOnly being resolved for some
   types.
 * LLVM backend: remove duplicate code for setting linkage and value
   name. The canonical place for this is in `updateDeclExports`.
 * LLVM backend: do some assembly template massaging to make `%%`
   rendered as `%`. More hacks will be needed to make inline assembly
   catch up with stage1.