cmd/cgo: for gccgo add #define to cgo_export.h for expected name

For gccgo we rename exported functions so that the compiler
will make them visible.  This CL adds a #define so that C
functions that #include "cgo_export.h" can use the expected
names of the function.

The test for this is the existing issue6833 test in
misc/cgo/test.  Without this CL it fails when using
-compiler=gccgo.

LGTM=minux.ma, rsc
R=golang-codereviews, gobot, rsc, minux.ma
CC=golang-codereviews
https://golang.org/cl/91830046

src/cmd/cgo/out.go

@@ -880,10 +880,24 @@ func (p *Package) writeGccgoExports(fgo2, fc, fm *os.File) {
 		fmt.Fprintf(cdeclBuf, ")")
 		cParams := cdeclBuf.String()
 
+		// We need to use a name that will be exported by the
+		// Go code; otherwise gccgo will make it static and we
+		// will not be able to link against it from the C
+		// code.
 		goName := "Cgoexp_" + exp.ExpName
 		fmt.Fprintf(fgcch, `extern %s %s %s __asm__("%s.%s");`, cRet, goName, cParams, gccgoSymbolPrefix, goName)
 		fmt.Fprint(fgcch, "\n")
 
+		// Use a #define so that the C code that includes
+		// cgo_export.h will be able to refer to the Go
+		// function using the expected name.
+		fmt.Fprintf(fgcch, "#define %s %s\n", exp.ExpName, goName)
+
+		// Use a #undef in _cgo_export.c so that we ignore the
+		// #define from cgo_export.h, since here we are
+		// defining the real function.
+		fmt.Fprintf(fgcc, "#undef %s\n", exp.ExpName)
+
 		fmt.Fprint(fgcc, "\n")
 		fmt.Fprintf(fgcc, "%s %s %s {\n", cRet, exp.ExpName, cParams)
 		fmt.Fprint(fgcc, "\t")