tags: include headers before source files

Currently looking up a structure definition in TAGS / tags takes one to
one of multiple "static struct X" definitions in arch sources, which makes
it for many structs practically impossible to get to the required header.
This patch changes the order of sources being tagged to first scan
architecture includes, then the top-level include/ directory, and only
then the rest. It also takes into account, that many architectures have
more than one include directory, i.e., not only arch/$ARCH/include, but
also arch/$ARCH/mach-X/include etc.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Reviewed-by: WANG Cong <xiyou.wangcong@gmail.com>
[mmarek@suse.cz: fix 'var+=text' bashism]
Signed-off-by: Michal Marek <mmarek@suse.cz>