allarch: Force TARGET_*FLAGS variable values

TARGET_CPPFLAGS, TARGET_CFLAGS, TARGET_CPPFLAGS and TARGET_LDFLAGS may
differ between MACHINEs. Since they are exported they affect task hashes
even if unused which leads to multiple variants of allarch packages
existing in sstate and bouncing in the sysroot when switching between
MACHINEs.

allarch packages shouldn't be using these variables anyway, so let's
ensure they have a fixed value in order to avoid this problem.

(Compare with 05a70ac3 and
14f4d016.)

Signed-off-by: Mike Crowe <mac@mcrowe.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>