libm: make 'neon' TARGET_ARCH_VARIANT a requirement for ARCH_ARM_HAVE_ARMV7A

This allows building on non-neon variants, because
ARCH_ARM_HAVE_ARMV7A := false was not being picked up, and required a TARGET_
to be put in front of it for it to be picked up.

Instead of making all devices specify the TARGET_ flag to pick up the
optimization let's condition it, since most of all supported devices
at this stage have neon capability.

Change-Id: Ia127f7b1cc594c22f077f956c2651f4c3a11f1ca