libstdc++: Deal with ENOSYS == ENOTSUP

zTPF uses the same numeric value for ENOSYS and ENOTSUP.

libstdc++-v3/ChangeLog:

2020-03-06  Andreas Krebbel  <krebbel@linux.ibm.com>

	* src/c++11/system_error.cc: Omit the ENOTSUP case statement if it
	would match ENOSYS.

libstdc++-v3/ChangeLog

+2020-03-06  Andreas Krebbel  <krebbel@linux.ibm.com>
+
+	* src/c++11/system_error.cc: Omit the ENOTSUP case statement if it
+	would match ENOSYS.
+
 2020-03-05  Jonathan Wakely  <jwakely@redhat.com>
 
 	* testsuite/27_io/filesystem/operations/all.cc: Mark unused variable.

libstdc++-v3/src/c++11/system_error.cc

@@ -251,7 +251,8 @@ namespace
 #ifdef ENOTSOCK
       case ENOTSOCK:
 #endif
-#ifdef ENOTSUP
+#if defined ENOTSUP && (!defined ENOSYS || ENOTSUP != ENOSYS)
+      // zTPF uses the same value for ENOSYS and ENOTSUP
       case ENOTSUP:
 #endif
 #ifdef ENOTTY