Improve shared data placement, require less atomic support and use unsafecell for the clocks

examples/stm32h755cm4/memory.x

@@ -9,6 +9,7 @@ SECTIONS
 {
     .ram_d3 :
     {
+        *(.ram_d3.shared_data)
         *(.ram_d3)
     } > RAM_D3
 }

examples/stm32h755cm4/src/bin/blinky.rs

@@ -10,7 +10,7 @@ use embassy_stm32::SharedData;
 use embassy_time::Timer;
 use {defmt_rtt as _, panic_probe as _};
 
-#[link_section = ".ram_d3"]
+#[link_section = ".ram_d3.shared_data"]
 static SHARED_DATA: MaybeUninit<SharedData> = MaybeUninit::uninit();
 
 #[embassy_executor::main]