cmd/cgo: stop using -fno-eliminate-unused-debug-types

This flag was added in January 2010, in CL 181102, to fix issue 497.
(Numbers were just shorter back then.) The fix was for OS X machines
and the llvm-gcc frontend.

In July 2011 we had to change the way we get enum values, because
there were no flags available to force Xcode's llvm-gcc to include the
enum names and values in DWARF debug output.

We now use clang, not llvm-gcc, on OS X machines.
Earlier versions of clang printed a warning about not knowing the flag.
Newer versions of clang now make that an error.

That is:
 - The flag was added for OS X machines.
 - The flag is no longer necessary on OS X machines.
 - The flag now breaks some OS X machines.

Remove it.

I have run the original program from issue 497 successfully
without the flag on both OS X and Linux machines.

Fixes #6678.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/18850043

src/cmd/cgo/gcc.go

@@ -742,13 +742,12 @@ func gccTmp() string {
 // the input.
 func (p *Package) gccCmd() []string {
 	c := append(p.gccBaseCmd(),
-		"-w",                                // no warnings
-		"-Wno-error",                        // warnings are not errors
-		"-o"+gccTmp(),                       // 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
+		"-w",          // no warnings
+		"-Wno-error",  // warnings are not errors
+		"-o"+gccTmp(), // write object to tmp
+		"-gdwarf-2",   // generate DWARF v2 debugging symbols
+		"-c",          // do not link
+		"-xc",         // input language is C
 	)
 	if strings.Contains(c[0], "clang") {
 		c = append(c,