This fixes a GDB internal error that may occur when the inferior has no
valid stack pointer in r15.
gdb/testsuite/ChangeLog:
* gdb.arch/s390-stackless.S: New.
* gdb.arch/s390-stackless.exp: New.
gdb/ChangeLog:
* s390-linux-tdep.c (s390_backchain_frame_unwind_cache): Avoid
exception when attempting to access the inferior's backchain.