Enable compilation of the newly added libdecnumber library code.
Object file targets are added to Makefile.target using a newly
introduced flag CONFIG_LIBDECNUMBER.  The flag is added
to the PowerPC targets (ppc[64]-linux-user, ppc[64]-softmmu).

Signed-off-by: Tom Musta <tommusta@gmail.com>
[agraf: add ppcemb and ppc64abi32 config]
Signed-off-by: Alexander Graf <agraf@suse.de>