linux-user: fix ssetmask() system call

linux-user: fix ssetmask() system call

Fix the ssetmask() system call by removing the invocation of sigorset().

The ssetmask() system call should replace the old signal mask
with the new and return the old mask. It shouldn't combine
the old and the new mask with sigorset(). Fetching the old
mask for sigorset() is also no longer needed.

The problem was detected after running LTP test group syscalls
for the MIPS EL 32 R2 architecture where the test ssetmask01 failed
with exit code 1. The test passes now that the ssetmask() system call
is fixed.

Signed-off-by: Miloš Stojanović <Milos.Stojanovic@rt-rk.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>