x86: Enforce toolchain to generate 64-bit codes for 64-bit U-Boot

x86: Enforce toolchain to generate 64-bit codes for 64-bit U-Boot

64-bit U-Boot image is a combination of 32-bit U-Boot (SPL) plus
64-bit U-Boot (proper). For the U-Boot proper, it has be compiled
to 64-bit object codes. Attempting to use a toolchain to compile
64-bit U-Boot for qemu-x86_64, like kernel.org 4.9 i386-linux-gcc,
fails with the following errors:

  arch/x86/cpu/intel_common/microcode.c:79:2: error: PIC register
  clobbered by 'ebx' in 'asm'

The issue is because toolchain is preconfigured to generate code
for the 32-bit architecture (i386), and currently '-m64' is missing
in the makefile fragment. Using kernel.org 4.9 x86_64-linux-gcc
works out of the box, since it is preconfigured to generate 64-bit
codes.

When compiling U-Boot SPL, '-m32' is passed to the toolchain, no
mater 32-bit (i386-linux-) or 64-bit (x86_64-linux) the toolchain
is preconfigured to generate.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>