build: remove _gcc_main.c during make clean

R=r, r2
CC=golang-dev
https://golang.org/cl/4226043

src/Make.pkg

@@ -48,7 +48,7 @@ coverage:
 	6cov -g $(shell pwd) $O.out | grep -v '_test\.go:'
 
 CLEANFILES+=*.cgo1.go *.cgo2.c _cgo_defun.c _cgo_gotypes.go _cgo_export.*
-CLEANFILES+=_cgo_.c _cgo_import.c _cgo_main.c _cgo_flags _cgo_run
+CLEANFILES+=_cgo_.c _cgo_import.c _cgo_main.c _cgo_flags _cgo_run _gcc_main.c
 CLEANFILES+=*.so _obj _test _testmain.go *.exe
 
 test: