s390x: Require GCC 7.1 or later to build glibc.

GCC 6.5 fails to correctly build ldconfig with recent ld.so.cache
commits, e.g.:
785969a0
elf: Implement a string table for ldconfig, with tail merging

If glibc is build with gcc 6.5.0:
__builtin_add_overflow is used in
<glibc>/elf/stringtable.c:stringtable_finalize()
which leads to ldconfig failing with "String table is too large".
This is also recognizable in following tests:
FAIL: elf/tst-glibc-hwcaps-cache
FAIL: elf/tst-glibc-hwcaps-prepend-cache
FAIL: elf/tst-ldconfig-X
FAIL: elf/tst-ldconfig-bad-aux-cache
FAIL: elf/tst-ldconfig-ld_so_conf-update
FAIL: elf/tst-stringtable

See gcc "Bug 98269 - gcc 6.5.0 __builtin_add_overflow() with small
uint32_t values incorrectly detects overflow"
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98269)