Add mips32 support to the cmake build.

Requires use of new cmake toolchain file:
$ cmake path/to/aom -DCMAKE_TOOLCHAIN_FILE=path/to/aom/build/cmake/toolchains/mips32-linux-gcc.cmake

DSPR2 and MSA are supported via addition of -DENABLE_DSPR2=1 and
-DENABLE_MSA=1 respectively. Note that the latter requires the addition
of -DMIPS_CPU=p5600.

BUG=https://bugs.chromium.org/p/aomedia/issues/detail?id=76

Change-Id: Idf7d7f2daecf18cc45b834166eaf34ee9f414d49
diff --git a/aom_scale/aom_scale.cmake b/aom_scale/aom_scale.cmake
index 3124bc7..a6aa31a 100644
--- a/aom_scale/aom_scale.cmake
+++ b/aom_scale/aom_scale.cmake
@@ -16,10 +16,19 @@
     "${AOM_ROOT}/aom_scale/generic/yv12extend.c"
     "${AOM_ROOT}/aom_scale/yv12config.h")
 
+set(AOM_SCALE_INTRIN_DSPR2
+    "${AOM_ROOT}/aom_scale/mips/dspr2/yv12extend_dspr2.c")
+
 # Creates the aom_scale build target and makes libaom depend on it. The libaom
 # target must exist before this function is called.
 function (setup_aom_scale_targets)
   add_library(aom_scale OBJECT ${AOM_SCALE_SOURCES})
-  set(AOM_LIB_TARGETS ${AOM_LIB_TARGETS} aom_scale PARENT_SCOPE)
   target_sources(aom PUBLIC $<TARGET_OBJECTS:aom_scale>)
+
+  if (HAVE_DSPR2)
+    add_intrinsics_object_library("" "dspr2" "aom_scale"
+                                  "AOM_SCALE_INTRIN_DSPR2")
+  endif ()
+
+  set(AOM_LIB_TARGETS ${AOM_LIB_TARGETS} aom_scale PARENT_SCOPE)
 endfunction ()