It seems the option is undefined for an arm64 build.

Trying to access it in a .gn file will result in the config error:

 target_cpu = "arm64" target_os = "android" use_goma = true compute_grit_inputs_for_analyze=true""" to /b/c/b/android_n5x_swarming_rel/src/out/Release/args.gn. /b/c/b/android_n5x_swarming_rel/src/buildtools/linux64/gn gen //out/Release --check -> returned 1 ERROR at //third_party/zlib/BUILD.gn:81:9: Undefined identifier if (arm_arch == "armv8-a+crc") { ^------- See //ui/gfx/BUILD.gn:255:5: which caused the file to be included. "//third_party/zlib", ^------------------- GN gen failed: 1

For an example, in a .gn file:
if (current_cpu == "arm64") {
   if (arm_arch == "armv8-a+crc") { }// error
}