Merge #652

652: Remove use of basepri register on thumbv8m.base r=AfoHT a=neonquill

The basepri register appears to be aviable on thumbv8m.main but not thumbv8m.base. At the very least, attempting to compile against a Cortex-M23 based Microchip ATSAML10E16A generates an error:

```
error[E0432]: unresolved import `cortex_m::register::basepri`
  --> /Users/dwatson/.cargo/registry/src/github.com-1ecc6299db9ec823/cortex-m-rtic-1.1.3/src/export.rs:25:5
   |
25 | use cortex_m::register::basepri;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ no `basepri` in `register`
```

I wasn't sure if it made more sense to replace the `armv7m` config flag with something related to basepri availability or to get closer to matching the cortex-m use of several architecture specific flags. In the end i chose to make the minimal change possible and just narrowed the existing `thumbv8m` check.

Context:
[cortex-m:src/register/mod.rs](https://github.com/rust-embedded/cortex-m/blob/4e908625204a1e95dd3fd5bdcd8d66d6bc11c3bc/src/register/mod.rs#L33):
```
#[cfg(all(not(armv6m), not(armv8m_base)))]
pub mod basepri;
```

[cortex-m:build.rs](https://github.com/rust-embedded/cortex-m/blob/4e908625204a1e95dd3fd5bdcd8d66d6bc11c3bc/build.rs#L21):
```
    } else if target.starts_with("thumbv8m.base") {
        println!("cargo:rustc-cfg=cortex_m");
        println!("cargo:rustc-cfg=armv8m");
        println!("cargo:rustc-cfg=armv8m_base");
```

Co-authored-by: David Watson <david@neonquill.com>

1 files changed, 12 insertions, 6 deletions

diff --git a/build.rs b/build.rs
index 7c518c90..ff9ebe35 100644
--- a/build.rs
+++ b/build.rs
@@ -7,15 +7,21 @@ fn main() {
         println!("cargo:rustc-cfg=rustc_is_nightly");
     }
 
-    if target.starts_with("thumbv6m") {
-        println!("cargo:rustc-cfg=armv6m");
-    }
-
+    // These targets all have know support for the BASEPRI register.
     if target.starts_with("thumbv7m")
         | target.starts_with("thumbv7em")
-        | target.starts_with("thumbv8m")
+        | target.starts_with("thumbv8m.main")
+    {
+        println!("cargo:rustc-cfg=have_basepri");
+
+    // These targets are all known to _not_ have the BASEPRI register.
+    } else if target.starts_with("thumb")
+        && !(target.starts_with("thumbv6m") | target.starts_with("thumbv8m.base"))
     {
-        println!("cargo:rustc-cfg=armv7m");
+        panic!(
+            "Unknown target '{}'. Need to update BASEPRI logic in build.rs.",
+            target
+        );
     }
 
     println!("cargo:rerun-if-changed=build.rs");