Bug 1324435 - mozboot: Install rust i686-msvc target. r=ted

Windows devs often want to target 32-bit windows. Make this
easier by installing the target for them at mozboot time.

MozReview-Commit-ID: 6gFbFBOqMz8

--HG--
extra : rebase_source : 12933c6cb7895cef859c9022efa87c62f321219e

python/mozboot/mozboot/mozillabuild.py

@@ -39,6 +39,9 @@ class MozillaBuildBootstrapper(BaseBootstrapper):
                 f.write('    PATH="$WIN_HOME/.cargo/bin:$PATH"\n')
                 f.write('    export PATH\n')
                 f.write('fi')
+            _, cargo_bin = self.cargo_home()
+            rustup = os.path.join(cargo_bin, 'rustup')
+            self.run([rustup, 'target', 'add', 'i686-pc-windows-msvc'])
         finally:
             try:
                 os.remove(rustup_init)