Make 32 bit builds via cmake possible on macos.

Working with ninja, make, and xcode generators using the
following args at cmake generation time:
-DAOM_TARGET_CPU=x86 -DCMAKE_OSX_ARCHITECTURES=i386 -DCONFIG_PIC=1

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

Change-Id: I830db21f7458417cb8048818e5681a78698a6068

build/cmake/aom_configure.cmake

@@ -30,18 +30,19 @@ if (NOT AOM_TARGET_CPU)
               "      CMAKE_GENERATOR=${CMAKE_GENERATOR}\n")
     endif ()
   endif ()
-  set(AOM_TARGET_SYSTEM ${CMAKE_SYSTEM_NAME})
+endif ()
-  if (NOT EXISTS "${AOM_ROOT}/build/cmake/targets/${AOM_TARGET_CPU}.cmake")
+
-    message("No RTCD template for ${AOM_TARGET_CPU}, using generic.")
+set(AOM_TARGET_SYSTEM ${CMAKE_SYSTEM_NAME})
+if (NOT EXISTS "${AOM_ROOT}/build/cmake/targets/${AOM_TARGET_CPU}.cmake")
+  message("No RTCD template for ${AOM_TARGET_CPU}, using generic.")
+  set(AOM_TARGET_CPU "generic")
+endif ()
+if ("${AOM_TARGET_CPU}" STREQUAL "x86" OR "${AOM_TARGET_CPU}" STREQUAL "x86_64")
+  find_program(YASM_EXECUTABLE yasm $ENV{YASM_PATH})
+  if (NOT YASM_EXECUTABLE)
+    message(WARNING "Unable to find yasm, using generic as target CPU.")
     set(AOM_TARGET_CPU "generic")
   endif ()
-  if (NOT AOM_TARGET_CPU STREQUAL "generic")
-    find_program(YASM_EXECUTABLE yasm $ENV{YASM_PATH})
-    if (NOT YASM_EXECUTABLE)
-      message(WARNING "Unable to find yasm, using generic as target CPU.")
-      set(AOM_TARGET_CPU "generic")
-    endif ()
-  endif ()
 endif ()
 
 include("${AOM_ROOT}/build/cmake/targets/${AOM_TARGET_CPU}.cmake")

build/cmake/aom_optimization.cmake

@@ -79,7 +79,9 @@ function (get_asm_obj_format out_format)
       message(FATAL_ERROR "Unknown obj format: ${AOM_TARGET_SYSTEM}")
     endif ()
   elseif ("${AOM_TARGET_CPU}" STREQUAL "x86")
-    if ("${AOM_TARGET_SYSTEM}" STREQUAL "Windows")
+    if ("${AOM_TARGET_SYSTEM}" STREQUAL "Darwin")
+      set(objformat "macho32")
+    elseif ("${AOM_TARGET_SYSTEM}" STREQUAL "Windows")
       set(objformat "win32")
     else ()
       message(FATAL_ERROR "Unknown obj format: ${AOM_TARGET_SYSTEM}")