runtime: extract Linux auxv handling

This refactoring is in preparation for handling auxv differently in
Android shared libraries.

Updates #18041.

Change-Id: If0458a309f9c804e7abd0a58b5a224d89f8da257
Reviewed-on: https://go-review.googlesource.com/33651
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/runtime/os_linux.go

@@ -200,6 +200,10 @@ func sysargs(argc int32, argv **byte) {
 
 	// now argv+n is auxv
 	auxv := (*[1 << 28]uintptr)(add(unsafe.Pointer(argv), uintptr(n)*sys.PtrSize))
+	sysauxv(auxv[:])
+}
+
+func sysauxv(auxv []uintptr) {
 	for i := 0; auxv[i] != _AT_NULL; i += 2 {
 		tag, val := auxv[i], auxv[i+1]
 		switch tag {