build-tools

This commit provides the build flags for Rust for AArch64. The core Rust
support already in the kernel does the rest. This enables the PAC ret
and BTI options in the Rust build flags to match the options that are
used when building C.

The Rust samples have been tested with this commit.

Signed-off-by: Jamie Cunliffe <Jamie.Cunliffe@arm.com>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Tested-by: Boqun Feng <boqun.feng@gmail.com>
Tested-by: Fabien Parent <fabien.parent@linaro.org>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Tested-by: Alice Ryhl <aliceryhl@google.com>