rustc_target: move in syntax::abi and flip dependency.

src/libsyntax/ast.rs

@@ -19,7 +19,7 @@ pub use util::parser::ExprPrecedence;
 
 use syntax_pos::{Span, DUMMY_SP};
 use codemap::{respan, Spanned};
-use abi::Abi;
+use rustc_target::spec::abi::Abi;
 use ext::hygiene::{Mark, SyntaxContext};
 use print::pprust;
 use ptr::P;