Add UDF instruction. Closes #199.

asm.s

@@ -114,6 +114,13 @@ __sev:
   sev
   bx lr
 
+
+  .section .text.__udf
+  .global __udf
+  .thumb_func
+__udf:
+  udf
+
   .section .text.__wfe
   .global __wfe
   .thumb_func

src/asm.rs

@@ -81,6 +81,35 @@ pub fn nop() {
     }
 }
 
+
+/// Generate an Undefined Instruction exception.
+///
+/// Can be used as a stable alternative to `core::intrinsics::abort`.
+#[inline]
+pub fn udf() -> ! {
+    match () {
+        #[cfg(all(cortex_m, feature = "inline-asm"))]
+        () => unsafe {
+            asm!("udf" :::: "volatile");
+            core::hint::unreachable_unchecked();
+        },
+
+        #[cfg(all(cortex_m, not(feature = "inline-asm")))]
+        () => unsafe {
+            extern "C" {
+                fn __udf();
+            }
+
+            __udf();
+
+            core::hint::unreachable_unchecked();
+        },
+
+        #[cfg(not(cortex_m))]
+        () => unimplemented!(),
+    }
+}
+
 /// Wait For Event
 #[inline]
 pub fn wfe() {