S/390: Fix mtune default.

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.ibm.com>

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

From-SVN: r262145