targets: aarch64-unknown-none: Add +strict-align

targets: aarch64-unknown-none: Add +strict-align

On AArch64, an unaligned access causes a synchronous exception. In the current
state of the target, the compiler might generate unaligned accesses, see
https://github.com/rust-embedded/rust-raspi3-tutorial/issues/10.

Since this is a bare-metal target, it is possible that there is no exception
handling in place (yet) to recover from this case, causing a binary to just
silently fail.

Add `+strict-align` to avoid this case.