Add @generated tag to libc syscall asm wrappers

Although libc syscall wrappers do not get checked in this can aid in
finding the source of generated files when spelunking in the objdir.

Multiple tools use @generated to identify generated files (for example,
in a review Phabricator will by default hide diffs in generated files).
For consistency use the @generated tag in makesyscalls.sh as we've done
for other generated files, even though these wrappers aren't checked in
to the tree.