PR16716 partial fix: Fix types and nesting for 'syscall.readahead'.

PR16716 partial fix: Fix types and nesting for 'syscall.readahead'.

* tapset/linux/nd_syscalls2.stp: Fix types, don't probe compat tasks.
* tapset/linux/syscalls2.stp: Likewise.
* tapset/linux/powerpc/nd_syscalls.stp: Add compat_readahead probes.
* tapset/linux/powerpc/syscalls.stp: Likewise.
* tapset/linux/s390/nd_syscalls.stp: Likewise.
* tapset/linux/s390/syscalls.stp: Likewise.
* tapset/linux/x86_64/nd_syscalls.stp: Likewise.
* tapset/linux/x86_64/syscalls.stp: Likewise.
* testsuite/buildok/nd_syscalls-arch-detailed.stp:
  Add compat_readahead testcase,
  Fix compat_ftruncate64 and compat_truncate64 testcases.
* testsuite/buildok/syscalls-arch-detailed.stp:
  Add compat_readahead testcase,
* testsuite/systemtap.syscall/readahead.c: New testcase.