libgfortran: Use __builtin_unreachable() not -Wno-stringop-overflow to silence warning

libgfortran: Use __builtin_unreachable() not -Wno-stringop-overflow to silence warning

The only caller of write_z is formatted_transfer_scalar_write that passes
kind to 'len'; in turn, write_z is the only caller of xtoa_big, passing on
its 'len'.  The kind is passed as is, except for GFC_REAL_17 for which
len = 16 is used.

libgfortran/
	* io/write.c (xtoa_big): Change a 'GCC diagnostic ignored
	"-Wstringop-overflow"' to an assumption (via __builtin_unreachable).t