S/390: Fix mtune default.

When building with --with-tune=zEC12 and calling the resulting gcc
with --march=z13 (no extra -mtune), the binary would unexpectedly be compiled
with -march=z13 -mtune=zEC12.  This patch avoids using the default tune
parameter if -march is specified as argument but the user can still
explicitly state -march=z13 -mtune=zEC12.

gcc/ChangeLog:

2018-06-26  Robin Dapp  <rdapp@linux.vnet.ibm.com>

	* config/s390/s390.h (enum processor_flags): Do not use
	default tune parameter when -march was specified.

From-SVN: r262143

gcc/ChangeLog

+2018-06-26  Robin Dapp  <rdapp@linux.vnet.ibm.com>
+
+        * config/s390/s390.h (enum processor_flags): Do not use
+        default tune parameter when -march was specified.
+
 2018-06-26  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/86314

gcc/config/s390/s390.h

@@ -205,7 +205,7 @@ enum processor_flags
 #define OPTION_DEFAULT_SPECS 					\
   { "mode", "%{!mesa:%{!mzarch:-m%(VALUE)}}" },			\
   { "arch", "%{!march=*:-march=%(VALUE)}" },			\
-  { "tune", "%{!mtune=*:-mtune=%(VALUE)}" }
+  { "tune", "%{!mtune=*:%{!march=*:-mtune=%(VALUE)}}" }
 
 #ifdef __s390__
 extern const char *s390_host_detect_local_cpu (int argc, const char **argv);