EmbeddedPkg/Library: Add VirtualRealTimeClockLib

EmbeddedPkg/Library: Add VirtualRealTimeClockLib

This is designed to be used on platforms where a a real RTC is not
available and relies on an RtcEpochSeconds variable having been set or,
if that is not the case, falls back to using the epoch embedded at
compilation time.

Note that, in order to keep things simple for the setting of the
compilation time variable, only GCC environments with UNIX-like shells
and where a 'date' command is available are meant to be supported for
now.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>