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>

generated/cr-buildbucket.cfg

diff --git a/generated/cr-buildbucket.cfg b/generated/cr-buildbucket.cfg
index d75f7ee..7ca8c1d 100644
--- a/generated/cr-buildbucket.cfg
+++ b/generated/cr-buildbucket.cfg
@@ -3764,6 +3764,10 @@
       }
       properties:
         '{'
+        '  "cmake_args": {'
+        '    "CMAKE_SYSTEM_NAME": "Windows",'
+        '    "CMAKE_SYSTEM_PROCESSOR": "arm64"'
+        '  },'
         '  "gclient_vars": {'
         '    "checkout_nasm": false'
         '  },'

main.star

diff --git a/main.star b/main.star
index ef0f389..fe8842d 100755
--- a/main.star
+++ b/main.star
@@ -1006,6 +1006,12 @@
     WIN_HOST,
     cq_enabled = False,
     properties = {
+        "cmake_args": {
+            # This is a cross-compile, so CMake needs to be told the processor.
+            # MSVC will pick up the architecture from msvc_target.
+            "CMAKE_SYSTEM_NAME": "Windows",
+            "CMAKE_SYSTEM_PROCESSOR": "arm64",
+        },
         "gclient_vars": {
             "checkout_nasm": False,
         },