PR18492: Add tapset support and test coverage for sched_{get,set}attr

PR18492: Add tapset support and test coverage for sched_{get,set}attr

* tapset/linux/aux_syscalls.stp: New function _struct_sched_attr_u()
* tapset/linux/nd_syscalls2.stp: New probes nd_syscall.sched_{g,s}etattr
* tapset/linux/syscalls2.stp: New probes syscall.sched_{g,s}etattr
* testsuite/buildok/aux_syscalls-embedded.stp: New subtest
* testsuite/buildok/nd_syscalls2-detailed.stp: Ditto
* testsuite/buildok/syscalls2-detailed.stp: Ditto
* testsuite/systemtap.syscall/sched_attr.c: New testcase