Use CMAKE_SIZEOF_VOID_P instead of CMAKE_CL_64

CMake's documentation says this is preferred.
https://cmake.org/cmake/help/latest/variable/CMAKE_CL_64.html

Reportedly, it also works better with MINGW, though we do not currently
support MINGW with the CMake build. See
https://boringssl-review.googlesource.com/c/boringssl/+/41704/

Change-Id: Ie5794306beeeff816b34ee98c7a0f8e0d4f99ec8
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/41724
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0867601..437870a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -471,7 +471,7 @@
   set(ARCH "x86_64")
 elseif(${CMAKE_SYSTEM_PROCESSOR} STREQUAL "AMD64")
   # cmake reports AMD64 on Windows, but we might be building for 32-bit.
-  if(CMAKE_CL_64)
+  if(CMAKE_SIZEOF_VOID_P EQUAL 8)
     set(ARCH "x86_64")
   else()
     set(ARCH "x86")

util/generate_build_files.py

diff --git a/util/generate_build_files.py b/util/generate_build_files.py
index 23d7ea5..623e7d3 100644
--- a/util/generate_build_files.py
+++ b/util/generate_build_files.py
@@ -489,7 +489,7 @@
   set(ARCH "x86_64")
 elseif(${CMAKE_SYSTEM_PROCESSOR} STREQUAL "AMD64")
   # cmake reports AMD64 on Windows, but we might be building for 32-bit.
-  if(CMAKE_CL_64)
+  if(CMAKE_SIZEOF_VOID_P EQUAL 8)
     set(ARCH "x86_64")
   else()
     set(ARCH "x86")