multimedia/aom: unbreak build on aarch64 after f0f115af

CMake Error at build/cmake/aom_configure.cmake:212 (enable_language):
  The CMAKE_ASM_COMPILER:

    as

  is not a full path and was not found in the PATH.

  Tell CMake where to find the compiler by setting either the environment
  variable "ASM" or the CMake cache entry CMAKE_ASM_COMPILER to the full
path
  to the compiler, or to the compiler name if it is in the PATH.

Reported by:	eduardo