[vm/runtime] Call tzset() before localtime_r().

POSIX doesn't guarantee that the timezone is set correctly before calling `localtime_r()`, so we need to call `tzset()` first to ensure that the timezone information is up-to-date.

As the glibc manual states:
> According to POSIX.1-2001, `localtime()` is required to behave as though `tzset(3)` was called, while `localtime_r()` does not
> have this requirement. For portable code, `tzset(3)` should be called before `localtime_r()`.

This change doesn't apply to Android, as from Android O and later Bionic libc handles timezone changes automatically.

Author: JesseRiemens

build/sanitizers/tsan_suppressions.cc

@@ -260,6 +260,10 @@ char kTSanDefaultSuppressions[] =
 // False positive in libc's tzset_internal, http://crbug.com/379738.
 "race:tzset_internal\n"
 
+// Suppression above does not work if we don't properly symbolize
+// tzset_internal frame. Suppress the closest caller.
+"race:dart::DN_HelperDateTime_timeZoneOffsetInSeconds\n"
+
 // http://crbug.com/380554
 "deadlock:g_type_add_interface_static\n"
 

runtime/vm/os_android.cc

@@ -102,6 +102,9 @@ intptr_t OS::ProcessId() {
 static bool LocalTime(int64_t seconds_since_epoch, tm* tm_result) {
   time_t seconds = static_cast<time_t>(seconds_since_epoch);
   if (seconds != seconds_since_epoch) return false;
+  // No need to call tzset() before localtime_r() because bionic 
+  // will handle timezone changes for us (starting from Android O).
+  // See https://android.googlesource.com/platform/bionic/+/ea87716696bf635706b6f3fa56b8a145add83aff
   struct tm* error_code = localtime_r(&seconds, tm_result);
   return error_code != nullptr;
 }

runtime/vm/os_linux.cc

@@ -418,6 +418,7 @@ intptr_t OS::ProcessId() {
 static bool LocalTime(int64_t seconds_since_epoch, tm* tm_result) {
   time_t seconds = static_cast<time_t>(seconds_since_epoch);
   if (seconds != seconds_since_epoch) return false;
+  tzset();  // Not guaranteed by POSIX to be called by `localtime_r`.
   struct tm* error_code = localtime_r(&seconds, tm_result);
   return error_code != nullptr;
 }