multilib/clsextend: Improve handling of regexps in PACKAGES_DYNAMIC

multilib/clsextend: Improve handling of regexps in PACKAGES_DYNAMIC

Now that PACKAGES_DYNAMIC is more standardised, starting with ^ anchors,
the variable manipulations performed by clsextend for multilib don't work.

This patch at least improves it to hack around the problem and enable
mulitlib builds to work again. If this code doesn't do the right thing, the
recipe is free to override the variable with the correct multilib case.

(From OE-Core rev: 593faec6e0155bdd7a43ee84c24de8ee20287681)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>