fail when the C compiler fails, and clean up when we do

compile-to-file returned process-wait's values and main dropped them,
so cc errors were printed, but then main compiler exited 0.

Also cleanup tmp C files when compilation failed.

tests/exit-code/hello.sex

@@ -0,0 +1,4 @@
+;;; Compiles cleanly, so the only way the build can fail is the bogus
+;;; flag handed to cc -- which is the point.
+(pub fn main () int
+  (return 0))