TimeZoneInfo: Reduce intermediate allocations in GetTimeZoneIds on Unix (dotnet/corec...

TimeZoneInfo: Reduce intermediate allocations in GetTimeZoneIds on Unix (dotnet/coreclr#8769)

Avoid intermediate underlying array allocations as items are added to
the resulting List<string> by specifying the capacity. Also, change
the signature of the private method to return List<string> instead of
IEnumerable<string> to avoid the enumerator allocation when enumerating
the ids.

Commit migrated from https://github.com/dotnet/coreclr/commit/0bca3c16e17f199bd8734aa1e2579e8d07ffe560