mk: honor TAGS for FILES in default group

Currently the lib32 crt files (/usr/lib32/Scrt1.o etc.) are placed in
the clibs-dev package rather than the clibs-dev-lib32 package.

The /usr/lib32/dtrace/drti.o file is similarly placed in the dtrace
package rather than the dtrace-lib32 package.

Splitting shared libraries in /usr/lib32 into a -lib32 package is
handled in bsd.lib.mk by adding "lib32" to TAGS. However bsd.files.mk
ignores TAGS and only honors ${group}TAGS since 144c442.

This patch changes the behavior of bsd.files.mk to honor TAGS if the
default FILES group is used. This matches the handling of PACKAGE, which
is also ignored unless the default FILES group is used.

With this patch, both the clibs and dtrace package are split correctly.

PR: 249145
Reviewed by: bapt
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46991