powerpc: Fix FPU post related link warnings

If we built POST on PPC's that didn't enable CONFIG_SYS_POST_FPU we'd
get the following warning with newer toolchains:

powerpc-linux-gnu-ld: Warning: lib_powerpc/fpu/libpostpowerpcfpu.o
		      uses hard float, libpost.o uses soft float

We actually worked around this sometime ago with the following commit:

commit ce82ff05
Author: Yuri Tikhonov <yur@emcraft.com>
Date:   Sat Dec 20 14:54:21 2008 +0300

   FPU POST: fix warnings when building with 2.18 binutils

However, this only took into effect if CONFIG_SYS_POST_FPU was enabled.
We can simply move the GNU_FPOST_ATTR out of the CONFIG_SYS_POST_FPU
ifdef block to address the issue.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>