bootstrap: set toolchain variables on per target basis

Using CC, CFLAGS, CXX, CXXFLAGS, AR and RANLIB breaks cross compilation
because host is built first and has correct values. The same
values are incorrect for the target however.

Author: mati865

src/bootstrap/builder.rs

@@ -1051,29 +1051,24 @@ impl<'a> Builder<'a> {
                 }
             };
             let cc = ccacheify(&self.cc(target));
-            cargo.env(format!("CC_{}", target), &cc).env("CC", &cc);
+            cargo.env(format!("CC_{}", target), &cc);
 
             let cflags = self.cflags(target, GitRepo::Rustc).join(" ");
             cargo
-                .env(format!("CFLAGS_{}", target), cflags.clone())
-                .env("CFLAGS", cflags.clone());
+                .env(format!("CFLAGS_{}", target), cflags.clone());
 
             if let Some(ar) = self.ar(target) {
                 let ranlib = format!("{} s", ar.display());
                 cargo
                     .env(format!("AR_{}", target), ar)
-                    .env("AR", ar)
-                    .env(format!("RANLIB_{}", target), ranlib.clone())
-                    .env("RANLIB", ranlib);
+                    .env(format!("RANLIB_{}", target), ranlib);
             }
 
             if let Ok(cxx) = self.cxx(target) {
                 let cxx = ccacheify(&cxx);
                 cargo
                     .env(format!("CXX_{}", target), &cxx)
-                    .env("CXX", &cxx)
-                    .env(format!("CXXFLAGS_{}", target), cflags.clone())
-                    .env("CXXFLAGS", cflags);
+                    .env(format!("CXXFLAGS_{}", target), cflags);
             }
         }