feat(riscv): avoid flash-backed LUT for irq

Previously, the `INTERRUPT_TO_PRIORITY` constant would compile down to a
runtime lookup table called `anon.HASH.2.llvm.NUMBERS` like so:

```asm
            status & configured_interrupts[INTERRUPT_TO_PRIORITY[cpu_intr as usize - 1]];
...
40380fbc:       3c0095b7                lui     a1,0x3c009
40380fc0:       2d858593                addi    a1,a1,728 # 3c0092d8 <anon.73967c570b981c3046610ed0622f71c3.2.llvm.13737670072012269642>
40380fc4:       952e                    add     a0,a0,a1
40380fc6:       4108                    lw      a0,0(a0)
```

Which means that in order to load the `a1`th slot from the LUT, we'll
have to go out to flash to do it.

As an alternative to an `#[inline]`'d function, which is nice in the
'c3's case because it's the identity function that disappears at
runtime, instead of using a `const` INTERRUPT_TO_PRIORITY could be made
a `static`, which would allow us to place it into the `.data` section
via a linker directive.

esp-hal-common/src/interrupt/riscv.rs

@@ -220,7 +220,7 @@ mod vectored {
 
         let configured_interrupts = get_configured_interrupts(crate::get_core(), status);
         let mut interrupt_mask =
-            status & configured_interrupts[INTERRUPT_TO_PRIORITY[cpu_intr as usize - 1]];
+            status & configured_interrupts[interrupt_to_priority(cpu_intr as usize)];
         while interrupt_mask != 0 {
             let interrupt_nr = interrupt_mask.trailing_zeros();
             // Interrupt::try_from can fail if interrupt already de-asserted:
@@ -606,6 +606,11 @@ mod classic {
     use super::{CpuInterrupt, InterruptKind, Priority};
     use crate::Cpu;
 
+    #[inline(always)]
+    pub(super) const fn interrupt_to_priority(irq: usize) -> usize {
+        irq
+    }
+
     pub(super) const PRIORITY_TO_INTERRUPT: [usize; 15] =
         [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15];