EmbeddedPkg: improve TimeBaseLib type safety

EmbeddedPkg: improve TimeBaseLib type safety

EfiTimeToEpoch currently returns a UINTN.
Also, some internal calculations was using UINTN for fixed-width.
Both of these lead to warnings/errors with VS2017.

Replace with appropriate fixed-size types.

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=997

Cc: Alexei Fedorov <Alexei.Fedorov@arm.com>
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>