ARM: improve VFP ABI check

Recent gcc versions define __ARM_PCS or __ARM_PCS_VFP to indicate the
VFP ABI in use, and ARM RVCT defines __SOFTFP__ when using this ABI.
If none of these are defined, check $cross_prefix and $cc for the
substring "hardfloat", and finally fall back to a linker test.  This
gives the correct result in most configurations.