Add generic implementation for muldi3 ...

Since version v2018.08.0 some shared copies of gcc routines got added to
barebox so that archs don't need to have their own copy inside their
lib. The arch I am working on atm also needs support for muldi3 which is
not present as a generic version right now.

This patch adds the generic version from latest linux v4.20 to barebox
and lets the archs select it in their Kconfig so they don't need to
provide it themself.

Signed-off-by: Martin Hofmann <martin.hofmann_at_mni.thm.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>