Rollup merge of rust-lang#48637 - segevfiner:restore-rust-mingw-download, r=alexcrichton

Fixes rust-lang#47311.
r? @nrc

Author: Manishearth

src/bootstrap/bootstrap.py

@@ -351,6 +351,14 @@ def download_stage0(self):
             with open(self.rustc_stamp(), 'w') as rust_stamp:
                 rust_stamp.write(self.date)
 
+            # This is required so that we don't mix incompatible MinGW
+            # libraries/binaries that are included in rust-std with
+            # the system MinGW ones.
+            if "pc-windows-gnu" in self.build:
+                filename = "rust-mingw-{}-{}.tar.gz".format(
+                    rustc_channel, self.build)
+                self._download_stage0_helper(filename, "rust-mingw")
+
         if self.cargo().startswith(self.bin_root()) and \
                 (not os.path.exists(self.cargo()) or
                  self.program_out_of_date(self.cargo_stamp())):