libdfp: Fix configure with compiler plus flags

This patch fix configure when issues defining a compiler as
path plus flags (for instance /my/compiler/gcc -m32). The downside
is configure with PATH to different toolchain will only works if the
same PATH is used when issuing make.
PowerPC: POWER8 build fixes

This patch fixes the power8/fpu imply (that was pointing to powerpc32
folder instead of powerpc64) and add it to dfp target to use
implementations at sysdep/powerpc/dfp.