Revert adding some arch-specific signal syscalls to <linux/syscalls.h>.

It turns out there is some variance on the calling conventions for
these syscalls, and <asm-generic/syscalls.h> is already the mechanism
used to handle this.  Switch arch/tile over to using that mechanism and
tweak the calling conventions for a couple of tile syscalls to match
<asm-generic/syscalls.h>.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>