PR18398: Tapset support for {get,set}_thread_area syscalls

PR18398: Tapset support for {get,set}_thread_area syscalls

* tapset/linux/i386/aux_syscalls.stp: New function _struct_user_desc_u()
* tapset/linux/i386/nd_syscalls.stp: Decode struct user_desc
* tapset/linux/i386/syscalls.stp: Ditto
* tapset/linux/x86_64/aux_syscalls.stp: New function _struct_user_desc_u()
* tapset/linux/x86_64/nd_syscalls.stp: New probes nd_syscall.{get,set}_thread_area
* tapset/linux/x86_64/syscalls.stp: New probes syscall.{get,set}_thread_area
* testsuite/buildok/nd_syscalls-arch-detailed.stp: New subtests
* testsuite/buildok/syscalls-arch-detailed.stp: New subtests
* testsuite/systemtap.syscall/thread_area.c: New testcase