First cut at floating-point environment functions that work with both
softfloat and vfp. This will be needed for places where people wish to use
vfp in a softfloat world, or to help moving to the softfp ABI.

With the softfp ABI the floating-point helpers may be written using the
existing softfloat code. Even if they are built on vfp existing
applications may be using the softfloat rounding values that will need to
be translated to vfp values.
Export _libc_arm_fpu_present to allow libm to determine if the vfp is
present or not.