x86, build: Build 16-bit code with -m16 where possible

x86, build: Build 16-bit code with -m16 where possible

Both clang 3.5 and GCC 4.9 will support this (as of r199754 and r207196
respectively). Both have been tested to produce booting kernels when the
16-bit code is built with -m16. (Modulo LLVM PR3997, at least.)

[ hpa: folded test for -m16 into M16_CFLAGS ]

Signed-off-by: David Woodhouse <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: H. Peter Anvin <[email protected]>