plan9: reduce the set of architecture-dependent files on Plan 9

From main repo: https://go-review.googlesource.com/#/c/2167

Change-Id: I2043458a432ccd2aa8b206adc107ac7b215e355f
Reviewed-on: https://go-review.googlesource.com/8720
Reviewed-by: Rob Pike <r@golang.org>

plan9/zsyscall_plan9_amd64.go

@@ -23,7 +23,7 @@ func fd2path(fd int, buf []byte) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
-func pipe(p *[2]_C_int) (err error) {
+func pipe(p *[2]int32) (err error) {
 	r0, _, e1 := Syscall(SYS_PIPE, uintptr(unsafe.Pointer(p)), 0, 0)
 	if int32(r0) == -1 {
 		err = e1