[vm] Ensure CpuInfo Windows flag detection works.

Fixes flutter/flutter#140138
TEST=manually on win x64 with `__ int3();` added in front of `roundsd` and forced `CpuId::sse41=false;`
Change-Id: I9e2dbf2615549dcc7c979036b5442276c2e0fb29
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/357218
Reviewed-by: Ryan Macnak <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

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();
   }