gcc-source: Fix incorrect task dependencies from ${B}

Some tasks may reference ${B} for gcc-source which in general would not exist.
It has dependencies on HOST_SYS and TARGET_SYS which are not appropriate for a
shared recipe like gcc-source. This causes problems for the archiver and
multiconfigs in particlar.

Set B to something else to avoid these task hash issues.

Acked-by: Jose Quaresma <jose.quaresma@foundries.io>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>