Improve gdb.base/float128.exp failure message

If the "print large128" sub-test fails in a manner that typically
indicates internal overflow due to GDB being built without MPFR
support, explicitly state this in the failure message.

gdb/testsuite/ChangeLog:
2018-06-20  Ulrich Weigand  <uweigand@de.ibm.com>

	* gdb.base/float128.exp: Add comment and improved fail message
	to the failure case of "print large128" test.