asf: build as 64-bit kernel module on e6500

asf: build as 64-bit kernel module on e6500

e6500 is built with 32b rootfs/64b kernel, build asf as 64bit too.

Fix the below build issue on e6500 core:
| DEBUG: Executing shell function do_make_scripts
| make: Entering directory
`.../tmp/sysroots/t4240qds/usr/src/kernel'
|   CC      scripts/mod/empty.o
| scripts/mod/empty.c:1:0: error: -mcmodel not supported in this
configuration
| make[2]: *** [scripts/mod/empty.o] Error 1
| make[1]: *** [scripts/mod] Error 2
| make: *** [scripts] Error 2

Signed-off-by: Ting Liu <b28495@freescale.com>