sh: Don't miss pending signals returning to user mode after signal processing

Without this patch, signals sent during architecture specific signal
handling (typically as a result of the user's stack being inaccessible)
are ignored.

This is the SH version of commit c3ff8ec3
which was for the i386.

Signed-off-by: Stuart Menefy <stuart.menefy@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>