Simply CMake assembly source selection.

Now that the naming for Apple platforms is aligned, we can select the
variable uniformly.

Change-Id: Id547d1a4ba0585d5f9e4ea0b5d8b255b2ab2ec38
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/51345
Reviewed-by: Adam Langley <agl@google.com>

util/generate_build_files.py

diff --git a/util/generate_build_files.py b/util/generate_build_files.py
index f1e4d98..3263d9b 100644
--- a/util/generate_build_files.py
+++ b/util/generate_build_files.py
@@ -586,11 +586,7 @@
             asm_files)
 
       cmake.write(
-R'''if(APPLE AND ARCH STREQUAL "aarch64")
-  set(CRYPTO_ARCH_SOURCES ${CRYPTO_apple_aarch64_SOURCES})
-elseif(APPLE AND ARCH STREQUAL "arm")
-  set(CRYPTO_ARCH_SOURCES ${CRYPTO_apple_arm_SOURCES})
-elseif(APPLE)
+R'''if(APPLE)
   set(CRYPTO_ARCH_SOURCES ${CRYPTO_apple_${ARCH}_SOURCES})
 elseif(UNIX)
   set(CRYPTO_ARCH_SOURCES ${CRYPTO_linux_${ARCH}_SOURCES})