Use two separate functions, depending on whether VFP/NEON is available.

This is set to require armv5te - it uses blx, which is only available
since armv5t, but we don't have a separate configure item for that.
(It also uses ldrd, which requires armv5te, but this could be avoided
if necessary.)

Signed-off-by: Martin Storsjö <martin@martin.st>