Differentiate AArch64 bare-metal targets between hf and non-hf.

Differentiate AArch64 bare-metal targets between hf and non-hf.

Following up on [1] and [2], this PR adds differntiation for aarch64 bare-metal
targets between versions with and without hardware floating point enabled.

This streamlines the target naming with other existing ARM targets and provides
the user clear indication if he is getting float or non-float for his bare-metal
target.

[1] https://github.com/rust-lang/rust/pull/60135#issuecomment-485851356
[2] https://github.com/rust-embedded/wg/issues/230

Closes: rust-embedded/wg#230