RISC-V support over CLINT (#815)

* Rebase to master

* using interrupt_mod

* bug fixes

* fix other backends

* Add changelog

* forgot about rtic-macros

* backend-specific configuration

* core peripherals optional over macro argument

* pre_init_preprocessing binding

* CI for RISC-V (WIP)

* separation of concerns

* add targets for RISC-V examples

* remove qemu feature

* prepare examples folder

* move examples all together

* move ci out of examples

* minor changes

* add cortex-m

* new xtask: proof of concept

* fix build.yml

* feature typo

* clean rtic examples

* reproduce weird issue

* remove unsafe code in user app

* update dependencies

* allow builds on riscv32imc

* let's fix QEMU

* Update .github/workflows/build.yml

Co-authored-by: Henrik Tjäder <henrik@tjaders.com>

* New build.rs

* removing test features

* adapt ui test to new version of clippy

* add more examples to RISC-V backend

* proper configuration of heapless for riscv32imc

* opt-out examples for riscv32imc

* point to new version of riscv-slic

* adapt new macro bindings

* adapt examples and CI to stable

* fix cortex-m CI

* Review

---------

Co-authored-by: Henrik Tjäder <henrik@tjaders.com>

examples/lm3s6965/examples/only-shared-access.rs

@@ -0,0 +1,43 @@
+//! examples/only-shared-access.rs
+
+#![no_main]
+#![no_std]
+#![deny(warnings)]
+#![deny(unsafe_code)]
+#![deny(missing_docs)]
+
+use panic_semihosting as _;
+
+#[rtic::app(device = lm3s6965, dispatchers = [UART0, UART1])]
+mod app {
+    use cortex_m_semihosting::{debug, hprintln};
+
+    #[shared]
+    struct Shared {
+        key: u32,
+    }
+
+    #[local]
+    struct Local {}
+
+    #[init]
+    fn init(_: init::Context) -> (Shared, Local) {
+        foo::spawn().unwrap();
+        bar::spawn().unwrap();
+
+        (Shared { key: 0xdeadbeef }, Local {})
+    }
+
+    #[task(shared = [&key])]
+    async fn foo(cx: foo::Context) {
+        let key: &u32 = cx.shared.key;
+        hprintln!("foo(key = {:#x})", key);
+
+        debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
+    }
+
+    #[task(priority = 2, shared = [&key])]
+    async fn bar(cx: bar::Context) {
+        hprintln!("bar(key = {:#x})", cx.shared.key);
+    }
+}