[gdb/testsuite] Add KFAILs in gdb.base/longjmp.exp

Add KFAILs in test-case gdb.base/longjmp.exp for PR gdb/26967, covering
various ways that gdb is unable to recover the longjmp target if the libc
probe is not supported.

Tested on x86_64-linux.

Approved-By: Simon Marchi <simon.marchi@efficios.com>