arch/arm: VFP and Thumb1 are not compatible

gcc will refuse to build with both --with-mode=thumb and --with-fpu=vfp,
with error messages during ./configure, like:

    checking for suffix of object files... configure: error: in `/home/ymor
    in/dev/buildroot/O/build/host-gcc-initial-4.9.3/build/arm-buildroot-lin
    ux-uclibcgnueabihf/libgcc':
    configure: error: cannot compute suffix of object files: cannot compile
    See `config.log' for more details.

And config.log informatively contains:

    sorry, unimplemented: Thumb-1 hard-float VFP ABI

This is an error message that comes deep from gcc source files.

If gcc says it does not support VFP with Thumb1, then let's disable that
combination in our menuconfig.

Prefer VFP over Thumb1, i.e. hide Thumb1 when we're not soft-float.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>