go-cross-canadian: set default value of DEBUG_PREFIX_MAP
It uses gcc on build machine during go-cross-canadian bootstrap, but the
gcc version may be old and not support option '-fmacro-prefix-map' which
is one of default values of DEBUG_PREFIX_MAP.
So set default value of DEBUG_PREFIX_MAP for go-cross-canadian.
Signed-off-by: Kai Kang <kai.kang@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>