PR18337: Test coverage improvements, tapset extension.

PR18337: Test coverage improvements, tapset extension.

* tapset/linux/aux_syscalls.stp: New auxiliary function _kexec_flags_str()
* tapset/linux/nd_syscalls.stp: Fix types and nesting for kexec_load
* tapset/linux/syscalls.stp: Likewise
* testsuite/buildok/nd_syscalls-detailed.stp: Test the flags_str variable
* testsuite/buildok/syscalls-detailed.stp: Likewise
* testsuite/systemtap.syscall/kexec_load.c: New testcase
* testsuite/systemtap.syscall/set_tid_address.c: New testcase