feat: support target-specific specialization of OPTEE_CLIENT_EXPORT

[TheButlah]

Closes #259

[ivila]

The targets aren’t limited to aarch64-unknown-linux-gnu and x86_64-unknown-linux-gnu. They can also include arm-unknown-linux-gnueabihf (see: arm-unknown-optee.json) and other variants, such as aarch64-unknown-none for no_std tests. It would be better to detect the architecture from the target triple rather than relying on the entire triple string.

[ivila]

Suggest using CARGO_CFG_TARGET_ARCH to significantly reduce the effort required here.

[ivila]

After reviewing the code, it seems the logic can be reduced to two parts.

First, there’s a check to ensure the target architecture is one of x86_64, aarch64, or arm.

Second, it reads the TARGET environment variable and constructs a Vec<String> containing OPTEE_CLIENT_EXPORT_{target}, OPTEE_CLIENT_EXPORT_{target_with_underscores}, and OPTEE_CLIENT_EXPORT, then checks whether any of them are defined in order.

The architecture check isn’t really necessary as developers generally know what they are doing, so I suggest we can focus directly on the second part instead.