runtime: support for AIX's procfs tree

On AIX, the process executable file is available under /proc/<pid>/object/a.out

Issue golang/go#19200

Change-Id: I26b6748c3f600cbcaa97ffa13d798e393fca780b
Reviewed-on: https://go-review.googlesource.com/37842
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: MatthieuSarter

libgo/go/os/executable_procfs.go

@@ -9,6 +9,7 @@ package os
 import (
 	"errors"
 	"runtime"
+	"syscall"
 )
 
 // We query the executable path at init time to avoid the problem of
@@ -27,6 +28,8 @@ var executablePath, executablePathErr = func() (string, error) {
 		procfn = "/proc/curproc/file"
 	case "dragonfly":
 		procfn = "/proc/curproc/file"
+	case "aix":
+		procfn = "/proc/" + itoa(syscall.Getpid()) + "/object/a.out"
 	}
 	return Readlink(procfn)
 }()