include/kernel.mk - better search for ARCH

If "findstring" is used without leading and trailing spaces unexpected matches
may happen. For example consider ARC=arc then "findstring $(ARCH)" will
report a false match with "aarch64".

But "findstring $ARCH " (note trailing space) will correctly skip
matches for both "aarch64" and "aarch64_be".

This patch is built-tested against NetGear WNDR3800.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Felix Fietkau <nbd@openwrt.org>
Cc: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 47359