[beta] [vm] Ensure CpuInfo Windows flag detection works.

BUG=flutter/flutter#140138
BUG=flutter/flutter#138784
BUG=flutter/flutter#138693
TEST=manually on win x64 with `__ int3();` added in front of `roundsd` and forced `CpuId::sse41=false;`

Cherry-pick: https://dart-review.googlesource.com/c/sdk/+/357218
Cherry-pick-request: #55211
Change-Id: I90a60d4972c5183d43516c628b68a71b7cc76912
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/357983
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>

runtime/vm/cpuinfo_win.cc

@@ -50,7 +50,11 @@ void CpuInfo::Cleanup() {
 
 bool CpuInfo::FieldContains(CpuInfoIndices idx, const char* search_string) {
   if (method_ == kCpuInfoCpuId) {
-    return CpuId::field(idx);
+    const char* field = CpuId::field(idx);
+    if (field == nullptr) return false;
+    bool contains = (strstr(field, search_string) != nullptr);
+    free(const_cast<char*>(field));
+    return contains;
   } else {
     UNREACHABLE();
   }