MdePkg: introduce DxeRuntimeDebugLibSerialPort

MdePkg: introduce DxeRuntimeDebugLibSerialPort

Introduce a variant of BaseDebugLibSerialPort that behaves correctly with
regards to the use of the serial port after ExitBootServices(). At boot
time, all DEBUG() prints and ASSERT() invocations are executed normally.
At runtime, DEBUG() prints are dropped entirely, and ASSERT()s omit the
serial output as well, and only perform the configured post-ASSERT()
action, i.e., issue a CPU breakpoint or enter a deadloop.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>