cgo: Use -fno-eliminate-unused-debug-types when calling gcc to for dwarf stabs.

This keeps debug symbols in for e.g. unreferenced enums.

Fixes #479

R=rsc
CC=golang-dev, waltermundt
https://golang.org/cl/181102

src/cmd/cgo/gcc.go

@@ -229,6 +229,7 @@ func (p *Prog) gccDebug(stdin []byte) (*dwarf.Data, string) {
 		"-Werror", // warnings are errors
 		"-o" + tmp, // write object to tmp
 		"-gdwarf-2", // generate DWARF v2 debugging symbols
+		"-fno-eliminate-unused-debug-types", // gets rid of e.g. untyped enum otherwise
 		"-c", // do not link
 		"-xc", // input language is C
 		"-", // read input from standard input