Rollup merge of rust-lang#38299 - achanda:ctrl-c, r=brson

Handle Ctrl+C in the build script

Author: frewsxcv

Diff for: x.py

@@ -16,4 +16,7 @@
 
 import bootstrap
 
-bootstrap.main()
+try:
+    bootstrap.main()
+except KeyboardInterrupt:
+    sys.exit()