ARM & AArch64 NEON: share the vabs implementation.

This changes ARM to use @llvm.fabs for floating-point vabs. Patterns
already existed in the backend, and it might help mid-end phases since
it's more likely to be understood than @llvm.arm.neon.vabs.
ARM: remove floating-point patterns for @llvm.arm.neon.vabs

The front-end is now generating the generic @llvm.fabs for this
operation now, so the extra patterns are no longer needed.