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`
```
This is an attempt to account for the fact that thumbv8m.base (M23)
MCUs don't have the BASEPRI register but have more than 32
interrupts. This moves away from the architecture specific config
flags and switches to a more functional flag.
Make the mask size depend on the max interrupt id
Rather than assuming a fixed interrupt count of 32 this code uses an
array of u32 bitmasks to calculate the priority mask. The size of this
array is calculated at compile time based on the size of the largest
interrupt id being used in the target code. For thumbv6m this should
be equivalent to the previous version that used a single u32 mask. For
thumbv8m.base it will be larger depending on the interrupts used.
Don't write 0s to the ISER and ICER registers
Writing 0s to these registers is a no-op. Since these masks should be
calculated at compile time, this conditional should result in writes
being optimized out of the code.
Prevent panic on non-arm targets
Panicking on unknown targets was breaking things like the doc build on
linux. This change should only panic when building on unknown arm
targets.
