cmake: -fmacro-prefix-map=${WEST_TOPDIR}=WEST_TOPDIR

Commit 28a5657f and f67dcdbd stopped ZEPHYR_BASE and
CMAKE_SOURCE_DIR from leaking into __FILE__ and other macros.
WEST_TOPDIR was still missing. Add it now that the new 'west topdir'
command has been implemented in
https://github.com/zephyrproject-rtos/west/pull/311



If the west version is too old then do nothing; same as before.

Any ASSERT in a zephyr module outside ZEPHYR_BASE is enough to test
this. One example:

$ZEPHYR_BASE/sanitycheck -T $ZEPHYR_BASE/tests/posix/fs/ -p qemu_x86 -v

readelf --string-dump=.rodata.disk_status.str1.1 {} \
  $(find sanity-out -name zfs_diskio.c.obj | head -n 1)

String dump of section '.rodata.disk_status.str1.1':

  [     0]  WEST_TOPDIR/modules/fs/fatfs/zfs_diskio.c
  [    2a]  pdrv < ((long) (((int) sizeof(char[1 - 2 * !(!__built....
  [    de]  ASSERTION FAIL [%s] @ %s:%d^J

Signed-off-by: Marc Herbert <marc.herbert@intel.com>