build: Get fixed address variables from 32bit compile pass (not 16bit)

Update the layoutrom.py build script so that fixed address sections
can come from the 32bit compiled C code.  Update the C code so that
all VAR16FIXED variables instead use the new VARFSEGFIXED which is
defined in 32bit mode.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
build: Minor - fix comments referring to old tools/ directory.

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