Add a cargo xbuild check on CI

.appveyor.yml

@@ -73,4 +73,5 @@ before_test:
   - cargo install-latest cargo-xbuild bootimage
 
 test_script:
+  - cargo xbuild
   - cargo bootimage

.travis.yml

@@ -33,5 +33,6 @@ before_script:
   - cargo install cargo-xbuild bootimage cargo-cache --debug -Z install-upgrade
 
 script:
+  - cargo xbuild
   - cargo bootimage
   - cargo cache --autoclean

azure-pipelines.yml

@@ -55,9 +55,12 @@ steps:
 - script: cargo install cargo-xbuild bootimage --debug
   displayName: 'Install cargo-xbuild and bootimage'
 
-- script: cargo bootimage
+- script: cargo xbuild
   displayName: 'Build'
 
+- script: cargo bootimage
+  displayName: 'Create Bootimage'
+
 - script: rustup component add rustfmt
   displayName: 'Install Rustfmt'