Document compatibility of the cm7-r0p1 feature.

The `cm7-r0p1` feature only works with the plateform feature `armv7em`.
I added a compile error to help someone figure that out more redily if
they accidentally combine those features.

Author: wt

cortex-m/src/lib.rs

@@ -94,6 +94,9 @@
 // Don't warn about feature(asm) being stable on Rust >= 1.59.0
 #![allow(stable_features)]
 
+#[cfg(all(feature = "cm7-r0p1", not(armv7em)))]
+compile_error!("The feature \"cm7-r0p1\" is only compatible with the armv7em target");
+
 extern crate bare_metal;
 extern crate volatile_register;