binutils: switch to 2.27 as the default version

Now that binutils 2.28 is available, switch to binutils 2.27 as the
default version, for both the host variant and the target variant. Note
that the target variant, when no host variant is built, was still
2.25.1: we forgot to update it to 2.26 when the host version was updated
to 2.26.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>