configure: arm: Check __ARM_PCS_VFP if the float ABI hasn't been defined

The support for detecting hardfp toolchains in af9dd50e is a
step in the right direction, but that particular switch case
isn't executed at all if the toolchain variable is set.

This fixes cross building from x86 ubuntu to armhf ubuntu,
in configurations such as
"CROSS=arm-linux-gnueabihf- ./configure --target=armv7-linux-gcc".

Change-Id: I6f141361e40374fbab564f9196d90e598fe9a0f6