[AArch64] Add precision choices for the reciprocal square root approximation

[Evandro Menezes]

        Add precision choices for the reciprocal square root approximation
        Allow a target to prefer such operation depending on the FP
   precision.

        gcc/
            * config/aarch64/aarch64-protos.h
            (AARCH64_EXTRA_TUNE_APPROX_RSQRT): New macro.
            * config/aarch64/aarch64-tuning-flags.def
            (AARCH64_EXTRA_TUNE_APPROX_RSQRT_DF): New mask.
            (AARCH64_EXTRA_TUNE_APPROX_RSQRT_SF): Likewise.
            * config/aarch64/aarch64.c
            (use_rsqrt_p): New argument for the mode.
            (aarch64_builtin_reciprocal): Devise mode from builtin.
            (aarch64_optab_supported_p): New argument for the mode.

This patch allows a target to choose for which FP precision the 
reciprocal square root approximation is used.

For example, though this approximation is improves the performance 
noticeably for DF on A57, for SF, not so much, if at all.

Feedback appreciated.

Thank you,

--
Evandro Menezes