x86, boot: use .code16gcc instead of .code16

Use .code16gcc to compile arch/x86/boot/bioscall.S rather than
.code16, since some older versions of binutils can't generate 32-bit
addressing expressions (67 prefixes) in .code16 mode, only in
.code16gcc mode.
Reported-by: NTetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>

arch/x86/boot/bioscall.S

@@ -13,7 +13,7 @@
  * touching registers they shouldn't be.
  */
 
-	.code16
+	.code16gcc
 	.text
 	.globl	intcall
 	.type	intcall, @function