Header file to produce 16-bit code with gcc

gcc for i386 can be used with the assembly prefix ".code16gcc" to generate
16-bit (real-mode) code.  This header file provides the assembly prefix.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>