MdePkg/BaseLib NASM Thunk16: Use NASM local labels

http://www.nasm.us/doc/nasmdoc3.html#section-3.9

A local label is a label beginning with the period, and it's actual
name is prefixed by the previous non-local label.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
MdePkg/BaseLib NASM Thunk16: Use bits 16 for 16-bit code

By using 'bits 16', we can write code for 16-bit use the actual
assembly syntax rather than 'DB' and sometimes writing code with
seemingly incorrect operands because we know it will run correctly
when the processor is running in 16-bit mode.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>