S/390: Support IBM z14 Model ZR1 with -march=native

S/390: Support IBM z14 Model ZR1 with -march=native

This adds the CPU model number of the IBM z14 Model ZR1 machine to
-march=native.  The patch doesn't actually change anything since we
anyway default to z14 for unknown CPU model numbers.  So this is just
for the sake of completeness.

2018-10-02  Andreas Krebbel  <krebbel@linux.ibm.com>

	* config/s390/driver-native.c (s390_host_detect_local_cpu): Add
	0x3907 as CPU model number.

From-SVN: r264797