cmd/link: external linking can fail on Solaris 11.2+

Workaround external linking issues encountered on Solaris 11.2+ due to
the go.o object file being created with a NULL STT_FILE symtab entry by
using a placeholder name.

Fixes #14957

Change-Id: I89c501b4c548469f3c878151947d35588057982b
Reviewed-on: https://go-review.googlesource.com/21636Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/link/internal/ld/symtab.go

@@ -198,7 +198,9 @@ func Asmelfsym() {
 
 	// Some linkers will add a FILE sym if one is not present.
 	// Avoid having the working directory inserted into the symbol table.
-	putelfsyment(0, 0, 0, STB_LOCAL<<4|STT_FILE, SHN_ABS, 0)
+	// It is added with a name to avoid problems with external linking
+	// encountered on some versions of Solaris. See issue #14957.
+	putelfsyment(putelfstr("go.go"), 0, 0, STB_LOCAL<<4|STT_FILE, SHN_ABS, 0)
 	numelfsym++
 
 	elfbind = STB_LOCAL