Avoid runtime relocation of 16bit "low" mem - calculate at build instead.

Avoid runtime relocation of 16bit "low" mem - calculate at build instead.

Some 16bit accesses to "low" mem variables use 16bit relocations
instead of the normal 32bit relocations.  This causes build problems
if the "low" mem sections move more than 64K during relocation.

The final location of the "low" memory can be determined during the
build, so link the 16bit code with the final post-reloc location of
the "low" mem variables instead.  This eliminates the need to do these
runtime relocations on the 16bit code.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>