scripts/tags.sh: use `find` for $ALLSOURCE_ARCHS generation

The current code includes 'Kconfig' in ALLSOURCE_ARCHS, but
it should not (Kconfig is not an architecture). Replace this
with a find-generated string and directly assign it to
$ALLSOURCE_ARCHS. The find_all_archs() function is no longer
needed for a one-liner with obvious semantics, so inline the
arch generation into the surrounding conditional.

Signed-off-by: Joey Pabalinas <joeypabalinas@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>