[RS6000] Fix non-linux powerpc breakage

svn r266496, git commit ab6b1bb456f broke non-linux powerpc builds
due to GNU_USER_DYNAMIC_LINKER being undefined.

	* config/rs6000/sysv4.h (GNU_USER_DYNAMIC_LINKER): Define.

From-SVN: r267292

gcc/ChangeLog

+2018-12-20  Alan Modra  <amodra@gmail.com>
+
+	* config/rs6000/sysv4.h (GNU_USER_DYNAMIC_LINKER): Define.
+
 2018-12-19  Andreas Tobler  <andreast@gcc.gnu.org>
 
 	* config.gcc: Enable TARGET_DEFAULT_ASYNC_UNWIND_TABLES on

gcc/config/rs6000/sysv4.h

@@ -761,6 +761,10 @@ GNU_USER_TARGET_CC1_SPEC
 #define MUSL_DYNAMIC_LINKER \
   "/lib/ld-musl-powerpc" MUSL_DYNAMIC_LINKER_E "%{msoft-float:-sf}.so.1"
 
+#ifndef GNU_USER_DYNAMIC_LINKER
+#define GNU_USER_DYNAMIC_LINKER GLIBC_DYNAMIC_LINKER
+#endif
+
 #define LINK_OS_LINUX_SPEC "-m elf32ppclinux %{!shared: %{!static: \
   %{rdynamic:-export-dynamic} \
   -dynamic-linker " GNU_USER_DYNAMIC_LINKER "}}"