sh: TS_RESTORE_SIGMASK conversion.

Replace TIF_RESTORE_SIGMASK with TS_RESTORE_SIGMASK and define our own
set_restore_sigmask() function.  This saves the costly SMP-safe set_bit
operation, which we do not need for the sigmask flag since TIF_SIGPENDING
always has to be set too.

Based on the x86 and powerpc change.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>