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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201313 91177308-0d34-0410-b5e6-96231b3b80d8