Add a section to place the veneers in memory

[hug-dev]

The veneers are for now only generated by the Arm GNU linker when it spots an entry function (one that was decorated with the cmse_nonsecure_entry attribute). Adding this section will allow to configure the SAU to make this section Non-Secure Callable.

Doing tests locally I could not see any warnings if this section was empty so I think this is fine. It is highly specific to the GNU toolchain so maybe you would want some preprocessing directive and cfg options which I am happy to add.

There is documentation for the section name at the end of this page.

It needs to be aligned on 32 bytes as a requirement from the Security Attribute Unit.

[rust-highfive]

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[hug-dev]

With this then one can do:

let peripherals = Peripherals::take().unwrap();
let mut sau = peripherals.SAU;

// Those symbols come from the linker file.
extern "C" {
    static mut __veneer_base: u32;
    static mut __veneer_limit: u32;
}
let base_nsc = unsafe { core::mem::transmute::<&mut u32, u32>(&mut __veneer_base) };
let limit_nsc = unsafe { core::mem::transmute::<&mut u32, u32>(&mut __veneer_limit) };

// Non-Secure Callable area
sau.set_region(2, SauRegion {
    base_address: base_nsc,
    limit_address: limit_nsc | 0x1F,
    attribute: SauRegionAttribute::NonSecureCallable,
}).unwrap();

sau.enable();

to configure its Non-Secure Callable section 🚀

[hug-dev]

Ok I did the change, I hope this is what you were expecting. I think this is fine to align everything to 4 bytes as veneers are 32 bits long (two thumb instructions). Only the start is aligned to 32 bytes because of SAU requirements.

[bors[bot]]

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