os: align siginfo argument to waitid

Backport https://golang.org/cl/46511 from gc trunk, as it may fix a
    bug reported for gccgo running on MIPS
    (https://groups.google.com/d/msg/golang-dev/sDg-t1_DPw0/-AJmLxgPBQAJ).
    
    Reviewed-on: https://go-review.googlesource.com/46571

From-SVN: r249599

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-f107cc8bced1939b0083231fc1ea24669ca4832c
+c49ba1ca392b3c23a4b3934e0a95a908b1dc2f1d
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/os/wait_waitid.go

@@ -23,7 +23,7 @@ func (p *Process) blockUntilWaitable() (bool, error) {
 	// On Darwin, it requires greater than or equal to 64 bytes
 	// for darwin/{386,arm} and 104 bytes for darwin/amd64.
 	// We don't care about the values it returns.
-	var siginfo [128]byte
+	var siginfo [16]uint64
 	psig := &siginfo[0]
 	_, _, e := syscall.Syscall6(syscall.SYS_WAITID, _P_PID, uintptr(p.Pid), uintptr(unsafe.Pointer(psig)), syscall.WEXITED|syscall.WNOWAIT, 0, 0)
 	runtime.KeepAlive(p)