AArch64: enable Cortex-A57 FP balancing on Cortex-A53.

Benchmarks have shown that it's harmless to the performance there, and having a
unified set of passes between the two cores where possible helps big.LITTLE
deployment.

Patch by Z. Zheng.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220744 91177308-0d34-0410-b5e6-96231b3b80d8