[libc] Add a redirecting <syscall.h> header. (#191069)
The amount of legacy code including `<syscall.h>` header instead of
`<sys/syscall.h>` (which is the regular header location on Linux
systems) out there is large.
Add a simple one-liner redirecting header to fix this compatibility
issues. In this PR I omit the regular licensing blurb at the top, given
the transient nature of this file, but I'm happy to add this if needed.
Also, given that it's effectively a compatibility shim, YAML generation
is not used.
