Use attributes for native module ABI and link name

Use attributes for native module ABI and link name

This patch changes how to specify ABI and link name of a native module.

Before:
  native "cdecl" mod llvm = "rustllvm" {...}

After:
  #[abi = "cdecl"]
  #[link_name = "rustllvm"]
  native mod llvm {...}

The old optional syntax for ABI and link name is no longer supported.

Fixes issue #547