Fix sigchain_test on mips

Pass correct size to syscall. Unlike on arm and x86,
_KERNEL__NSIG=128 on mips.

This fixes the test on mips32 and mips64.

Test: mma test-art-target-gtest-sigchain_test in QEMU

Change-Id: I3acd93f3595bcbd2a9a0dc3248955226a1051745
1 file changed