MdePkg BaseLib NASM Thunk16: Initialize _16GdtrBase to 0

Previously we were setting it to the start of the GDT in the UEFI
driver's copy of BaseLib. This address is irrelavent, since the C code
will initialize it to the cooresponding address after the Thunk16 code
has been placed in low memory.

Additionally, initializing this was causing a link error for the
XCLANG toolchain.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Cc: Sergey Isakov <isakov-sl@bk.ru>