fix libc-internal signal blocking on mips archs

fix libc-internal signal blocking on mips archs

due to historical reasons, the mips signal set has 128 bits rather
than 64 like on every other arch. this was special-cased correctly, at
least for 32-bit mips, at one time, but was inadvertently broken in
commit 7c440977db9444d7e6b1c3dcb1fdf4ee49ca4158, and seems never to
have been right on mips64/n32.

as consequenct of this bug, applications making use of high realtime
signal numbers on mips may have been able to execute application code
in contexts where doing so was unsafe.