x86/build: Remove -m16 workaround for unsupported versions of GCC

Revert the following two commits:

  de3accda ("x86, build: Build 16-bit code with -m16 where possible")
  a9cfccee ("x86, build: Change code16gcc.h from a C header to an assembly header")

Since

  0bddd227 ("Documentation: update for gcc 4.9 requirement")

the minimum supported version of GCC is gcc-4.9. It's now safe to remove
this code.

Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lkml.kernel.org/r/20201201011307.3676986-1-ndesaulniers@google.com