Keith,
May I ask if the SIMD and VFP extensions are enabled before you call the floating point functions please?
The A15 Technical Reference Manual (TRM) mentions "From reset, both the Advanced SIMD and VFP extensions are disabled. Any attempt to execute either an Advanced SIMD or VFP instruction results in an Undefined Instruction exception being taken."
You can enable the CP10 and CP11 access in Coprocessor control register as mentioned in the A15 TRM document, which should be accessible in ARM website.