This syscall was added to simplify the the libc implementations of
fchmodat, as the original syscall does not take a `flags` argument.
Another syscall, `map_shadow_stack`, was also added for x86_64.
Previously, updating the `SYS` enum for each architecture required
manually looking at the syscall tables and inserting any new additions.
This commit adds a tool, `generate_linux_syscalls.zig`, that automates
this process using the syscall tables in the Linux source tree. On
architectures without a table, it runs `zig cc` as a pre-processor to
extract the system-call numbers from the Linux headers.
