in pipe2, use pipe() rather than __syscall(SYS_pipe, ...) for fallback

in pipe2, use pipe() rather than __syscall(SYS_pipe, ...) for fallback

SYS_pipe is not usable directly in general, since mips has a very
broken calling convention for the pipe syscall. instead, just call the
function, so that the mips-specific ugliness is isolated in
mips/pipe.s and not copied elsewhere.