Add basic arm64 linux support to the cmake build.

Requires new cmake toolchain file arm64-linux-gcc.cmake:
$ cmake path/to/aom
-DCMAKE_TOOLCHAIN_FILE=path/to/aom/build/cmake/toolchains/arm64-linux-gcc.cmake

Above requires aarch64-linux-gnu-gcc toolchain is installed and its
bin dir added to PATH.

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

Change-Id: I6c7e297a67415008583b7ac2cd8aea528787e6d0

build/cmake/aom_configure.cmake

@@ -98,9 +98,8 @@ elseif ("${AOM_TARGET_CPU}" MATCHES "arm")
     set(AS_EXECUTABLE as)
     set(AOM_AS_FLAGS -arch ${AOM_TARGET_CPU} -isysroot ${CMAKE_OSX_SYSROOT})
   elseif ("${AOM_TARGET_SYSTEM}" STREQUAL "Linux")
-    if (NOT AS_EXECUTABLE)
-      message(FATAL_ERROR "Assembler missing for ${AOM_TARGET_CPU}-Linux.")
-    endif ()
+    # arm linux assembler settings controlled by
+    # build/cmake/toolchains/arm*-linux*.cmake
   endif ()
   if (NOT AS_EXECUTABLE)
     message(FATAL_ERROR

build/cmake/toolchains/arm64-linux-gcc.cmake

@@ -0,0 +1,31 @@
+##
+## Copyright (c) 2017, Alliance for Open Media. All rights reserved
+##
+## This source code is subject to the terms of the BSD 2 Clause License and
+## the Alliance for Open Media Patent License 1.0. If the BSD 2 Clause License
+## was not distributed with this source code in the LICENSE file, you can
+## obtain it at www.aomedia.org/license/software. If the Alliance for Open
+## Media Patent License 1.0 was not distributed with this source code in the
+## PATENTS file, you can obtain it at www.aomedia.org/license/patent.
+##
+if (NOT AOM_BUILD_CMAKE_TOOLCHAINS_ARM64_LINUX_GCC_CMAKE_)
+set(AOM_BUILD_CMAKE_TOOLCHAINS_ARM64_LINUX_GCC_CMAKE_ 1)
+
+set(CMAKE_SYSTEM_NAME "Linux")
+
+# TODO(tomfinegan): Allow control of compiler prefix (aka $CROSS).
+set(CMAKE_C_COMPILER aarch64-linux-gnu-gcc)
+set(CMAKE_CXX_COMPILER aarch64-linux-gnu-g++)
+set(AS_EXECUTABLE arm-linux-gnueabihf-as)
+set(CMAKE_C_COMPILER_ARG1 "-march=armv8-a")
+set(CMAKE_CXX_COMPILER_ARG1 "-march=armv8-a")
+set(AOM_AS_FLAGS "-march=armv8-a")
+set(CMAKE_SYSTEM_PROCESSOR "arm64")
+
+# No intrinsics flag required for arm64-linux-gcc.
+set(AOM_NEON_INTRIN_FLAG "")
+
+# No runtime cpu detect for arm64-linux-gcc.
+set(CONFIG_RUNTIME_CPU_DETECT 0 CACHE BOOL "")
+
+endif ()  # AOM_BUILD_CMAKE_TOOLCHAINS_ARM64_LINUX_GCC_CMAKE_