fix regression that negated some mips syscall error returns

fix regression that negated some mips syscall error returns

due to what was essentially a copy and paste error, the changes made
in commit f61be1f875a2758509d6e9e2cf6f1d9603b28b65 caused syscalls
with 5 or 6 arguments (and syscalls with 2, 3, or 4 arguments when
compiled with clang compatibility) to negate the returned error code a
second time, breaking errno reporting.