LTTng system call tracing

1) lttng-syscall-extractor

You need to build a kernel with CONFIG_FTRACE_SYSCALLS=y and
CONFIG_KALLSYMS_ALL=y for extraction. Apply the linker patch to get your
kernel to keep the system call metadata after boot.  Then build and load
the LTTng syscall extractor module. The module will fail to load (this
is expected). See the dmesg output for system call metadata.

2) Generate system call TRACE_EVENT().

Take the dmesg data and feed it to lttng-syscalls-generate-headers.sh from the
tools/syscalls directory. See the script header for usage example. It should be
run for both the integers and pointers types.

After these are created, you just need to follow the new system call additions,
no need to regenerate the whole thing, since system calls are only appended to.

3) Override headers

You need to provide override headers (even if they don't contain
overrides) for each architecture. Those are meant to contain manual
overrides for specific system calls for cases where the script do not
generate the correct typing for specific system calls.

4) Integrating the new architecture, updating architecture header

Edit instrumentation/syscalls/syscalls_integers.h and
instrumentation/syscalls/syscalls_pointers.h. Add the new architecture (e.g.
CONFIG_X86_64) to the newly-generated headers (e.g. changing the current
x86-64-syscalls_<CLASS>.h into x86-64-syscalls_<CLASS>.h), then rebuild
lttng-modules.