Improve MinGW detection when cross compiling

src/bootstrap/dist.rs

@@ -234,7 +234,14 @@ fn make_win_dist(
         }
     }
 
-    let target_tools = ["gcc.exe", "ld.exe", "dlltool.exe", "libwinpthread-1.dll"];
+    let compiler = if target_triple == "i686-pc-windows-gnu" {
+        "i686-w64-mingw32-gcc.exe"
+    } else if target_triple == "x86_64-pc-windows-gnu" {
+        "x86_64-w64-mingw32-gcc.exe"
+    } else {
+        "gcc.exe"
+    };
+    let target_tools = [compiler, "ld.exe", "dlltool.exe", "libwinpthread-1.dll"];
     let mut rustc_dlls = vec!["libwinpthread-1.dll"];
     if target_triple.starts_with("i686-") {
         rustc_dlls.push("libgcc_s_dw2-1.dll");

src/librustc_target/spec/i686_pc_windows_gnu.rs

@@ -5,6 +5,7 @@ pub fn target() -> TargetResult {
     base.cpu = "pentium4".to_string();
     base.max_atomic_width = Some(64);
     base.eliminate_frame_pointer = false; // Required for backtraces
+    base.linker = Some("i686-w64-mingw32-gcc".to_string());
 
     // Mark all dynamic libraries and executables as compatible with the larger 4GiB address
     // space available to x86 Windows binaries on x86_64.

src/librustc_target/spec/x86_64_pc_windows_gnu.rs

@@ -5,6 +5,7 @@ pub fn target() -> TargetResult {
     base.cpu = "x86-64".to_string();
     base.pre_link_args.get_mut(&LinkerFlavor::Gcc).unwrap().push("-m64".to_string());
     base.max_atomic_width = Some(64);
+    base.linker = Some("x86_64-w64-mingw32-gcc".to_string());
 
     Ok(Target {
         llvm_target: "x86_64-pc-windows-gnu".to_string(),