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 metadata and feed it to lttng-syscalls-generate-headers.sh, e.g.,
from the instrumentation/syscalls directory. See the script header for
usage example. It should be run for both the integers and pointers types.

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