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: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>