[gardening] Fix MSVC/arm64 build

MSVC/arm64 cross-build on x64 fails with errors

runtime\platform\unwinding_records.h(256): error C2607: static assertion failed

It is incorrect to validate consistency between O/S structure (host
declaration) and Dart VM structure (target declaration) in case of
a mismatch between host and target.

TEST=ci

Change-Id: Icd2729656ce51088010c192308733896ce40a1b1
Cq-Include-Trybots: luci.dart.try:vm-msvc-win-arm64-try,vm-msvc-win-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/463401
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

runtime/platform/unwinding_records.h

@@ -47,6 +47,11 @@ typedef uint32_t DWORD;
 #endif
 
 #if defined(TARGET_ARCH_X64)
+
+#if !defined(HOST_ARCH_X64)
+#define UNWINDING_RECORDS_HOST_TARGET_MISMATCH 1
+#endif
+
 //
 // Refer to https://learn.microsoft.com/en-us/cpp/build/exception-handling-x64
 //
@@ -120,6 +125,10 @@ struct CodeRangeUnwindingRecord {
 
 #elif defined(TARGET_ARCH_ARM64)
 
+#if !defined(HOST_ARCH_ARM64)
+#define UNWINDING_RECORDS_HOST_TARGET_MISMATCH 1
+#endif
+
 // ARM64 unwind codes are defined in below doc.
 // https://docs.microsoft.com/en-us/cpp/build/arm64-exception-handling#unwind-codes
 enum UnwindOp8Bit {
@@ -252,7 +261,10 @@ struct CodeRangeUnwindingRecord {
 //
 // Make sure that the sizes match, so that TargetRuntimeFunction values
 // can be used as RUNTIME_FUNCTION values and vice versa in the runtime.
-#if defined(UNWINDING_RECORDS_WINDOWS_HOST)
+// The check doesn't make sense when host and target platforms are different
+// though, and no code depends on it in such cross-platform scenario.
+#if defined(UNWINDING_RECORDS_WINDOWS_HOST) &&                                 \
+    !defined(UNWINDING_RECORDS_HOST_TARGET_MISMATCH)
 static_assert(sizeof(TargetRuntimeFunction) == sizeof(RUNTIME_FUNCTION));
 #endif