Specify CMAKE_SYSTEM_PROCESSOR on win_arm64_msvc_compile

Otherwise, CMake thinks it's building for the host and our build gets a
little confused. (It's actually *mostly* fine, but the logic to decide
whether to use NASM will trip up.)

Change-Id: Iac411eab8da5a9b8283e7daba649bdba7d2ef586
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/57945
Reviewed-by: Bob Beck <bbe@google.com>