[gdb/testsuite] Use maint ignore-probes in gdb.base/longjmp.exp

Test-case gdb.base/longjmp.exp handles both the case that there is a libc
longjmp probe, and the case that there isn't.

However, it only tests one of the two cases.

Use maint ignore-probes to test both cases, if possible.

Tested on x86_64-linux.