Bionic/libm: Prefer branches and VFP ABI.

For internal functions set gcc attribute "aapcs-vfp" for ARM
and use -fno-if-conversion to prefer branches over predicated
instructions (improves performance on architectures with good
branch prediction).

Change-Id: I8424e0e82a19d35e7e3b6e3e122dcdecdd5426fd