libvpx: Add VP9 decoder.

source/libvpx/vp9/common/ppc/vp9_platform_altivec.asm

+;
+;  Copyright (c) 2010 The WebM project authors. All Rights Reserved.
+;
+;  Use of this source code is governed by a BSD-style license
+;  that can be found in the LICENSE file in the root of the source
+;  tree. An additional intellectual property rights grant can be found
+;  in the file PATENTS.  All contributing project authors may
+;  be found in the AUTHORS file in the root of the source tree.
+;
+
+
+    .globl save_platform_context
+    .globl restore_platform_context
+
+.macro W V P
+    stvx    \V,  0, \P
+    addi    \P, \P, 16
+.endm
+
+.macro R V P
+    lvx     \V,  0, \P
+    addi    \P, \P, 16
+.endm
+
+;# r3 context_ptr
+    .align 2
+save_platform_contex:
+    W v20, r3
+    W v21, r3
+    W v22, r3
+    W v23, r3
+    W v24, r3
+    W v25, r3
+    W v26, r3
+    W v27, r3
+    W v28, r3
+    W v29, r3
+    W v30, r3
+    W v31, r3
+
+    blr
+
+;# r3 context_ptr
+    .align 2
+restore_platform_context:
+    R v20, r3
+    R v21, r3
+    R v22, r3
+    R v23, r3
+    R v24, r3
+    R v25, r3
+    R v26, r3
+    R v27, r3
+    R v28, r3
+    R v29, r3
+    R v30, r3
+    R v31, r3
+
+    blr