Revert "Remove cortex-m-semihosting CI"

This reverts commit f4c48301dd3c8013e03d8e4748613e6fe17a7d93.

It has been decided that the CI will be left in for now, and updated to
work with this crate in the future.

ci/install.sh

@@ -0,0 +1,12 @@
+set -euxo pipefail
+
+main() {
+    if [ $TARGET = thumbv7m-none-eabi ]; then
+        cargo install --list | grep xargo || \
+            cargo install xargo
+        rustup component list | grep 'rust-src.*installed' || \
+            rustup component add rust-src
+    fi
+}
+
+main

ci/script.sh

@@ -0,0 +1,14 @@
+set -euxo pipefail
+
+main() {
+    local cargo=
+    if [ $TARGET = thumbv7m-none-eabi ]; then
+        cargo=xargo
+    else
+        cargo=cargo
+    fi
+
+    $cargo check --target $TARGET
+}
+
+main