Noticed recently that `FEXServer -w` was broken and couldn't understand
why. Turns out that FHU syscall handling was /always/ falling down the
`#else` path in the handlers since cmake `add_definitions` follows
folder scoping rules.
This means it was always returning -1, which was causing FEXServer's
pidfd_open usage to always receive -1, which meant the sendmsg with FD
was always failing, which meant the `FEXServer -w` would forever wait
for a message that was never sent.
Converting the utility over to a target not only fixes definition
scoping problems, but also makes the other paths actually work.
This found some compiling bugs and instead lets us define SYS_pidfd_open
if it doesn't exist. Letting the kernel return the ENOSYS if it doesn't
exist on that platform.
Main thing, fixes FEXServer -w hanging forever.
Adds a header only include utility folder that can be included from
everywhere.
Contains syscall helpers for older glibc and defines for older Linux
uapi headers missing some defines.
