x86, build: Change code16gcc.h from a C header to an assembly header

# that way we can complain to the user if the CPU is insufficient.

#

# The -m16 option is supported by GCC >= 4.9 and clang >= 3.5. For

# older versions of GCC, we need to play evil and unreliable tricks to

# attempt to ensure that our asm(".code16gcc") is first in the asm

# output.

CODE16GCC_CFLAGS := -m32 -include $(srctree)/arch/x86/boot/code16gcc.h \

		    $(call cc-option, -fno-toplevel-reorder,\

		      $(call cc-option, -fno-unit-at-a-time))

# older versions of GCC, include an *assembly* header to make sure that

# gcc doesn't play any games behind our back.

CODE16GCC_CFLAGS := -m32 -Wa,$(srctree)/arch/x86/boot/code16gcc.h

M16_CFLAGS	 := $(call cc-option, -m16, $(CODE16GCC_CFLAGS))

REALMODE_CFLAGS	:= $(M16_CFLAGS) -g -Os -D__KERNEL__ \

/*

 * code16gcc.h

 *

 * This file is -include'd when compiling 16-bit C code.

 * Note: this asm() needs to be emitted before gcc emits any code.

 * Depending on gcc version, this requires -fno-unit-at-a-time or

 * -fno-toplevel-reorder.

 *

 * Hopefully gcc will eventually have a real -m16 option so we can

 * drop this hack long term.

 */

#

# code16gcc.h

#

# This file is added to the assembler via -Wa when compiling 16-bit C code.

# This is done this way instead via asm() to make sure gcc does not reorder

# things around us.

#

# gcc 4.9+ has a real -m16 option so we can drop this hack long term.

#

#ifndef __ASSEMBLY__

asm(".code16gcc");

#endif

	.code16gcc