Use macros for .code16/32 mode switches in inline asm in stacks.c

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
Eliminate FUNCFSEG - only force portions of inline asm to f-segment

The FUNCFSEG macro was introduced to force a C function into the
f-segment.  This was needed for some C functions that used inline
assembler that contained some 16bit code.  Instead of forcing the
entire C function into the f-segment, just force the small subset of
inline assembler into the f-segment.

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