Macro libc_stubs 

macro_rules! libc_stubs {
    () => { ... };
}

Provide the libc stubs a bare-metal newlib link expects, sized for this crate’s allocation model.

Emits _sbrk returning failure: the IREE runtime allocates exclusively from the Arena, so the libc heap must never grow. Invoke once at module scope in the firmware binary:

iree_embedded::libc_stubs!();