.gitlab-ci.yml: added cross compilation and tests on mips/aarch64/arm

This utilizes the qemu-user system used by gnutls. This also
deprecates the previous aarch64 build.
Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>

.gitlab-ci.yml

 variables:
   BUILD_IMAGES_PROJECT: gnutls/build-images
+  DEBIAN_CROSS_BUILD: buildenv-debian-cross
   FEDORA_BUILD: buildenv-f28
   FEDORA_X86_BUILD: buildenv-f28-x86
   GET_SOURCES_ATTEMPTS: "3"
@@ -29,19 +30,6 @@ build/x86:
   - shared
   except:
   - tags
-build/aarch64:
-  before_script:
-  - /bin/true
-  script:
-  - ./.bootstrap &&
-    ./configure --enable-fat --disable-documentation && make -j4 &&
-    make check -j4
-  tags:
-  - aarch64
-  only:
-  - branches@gnutls/nettle
-  except:
-  - tags
 build/mini-gmp:
   image: $CI_REGISTRY/$BUILD_IMAGES_PROJECT:$FEDORA_BUILD
   script:
@@ -110,3 +98,30 @@ build/gnutls:
       - gnutls-git/*.log
       - gnutls-git/tests/*/*.log
       - gnutls-git/tests/suite/*/*.log
+.Debian.cross.template: &Debian_cross_template
+  image: $CI_REGISTRY/$BUILD_IMAGES_PROJECT:$DEBIAN_CROSS_BUILD
+  before_script:
+  # remove any previously installed nettle headers to avoid conflicts
+  - for arch in armhf mips arm64;do apt-get remove -y nettle-dev:$arch;done
+  script:
+  - build=$(dpkg-architecture -qDEB_HOST_GNU_TYPE)
+  - host="${CI_JOB_NAME#*.cross.}"
+  # not setting CC_FOR_BUILD paired with qemu-user/binfmt somehow causes
+  # config.guess to detect the target as the build platform and not activate
+  # cross-compile mode even though --build is given
+  - export CC_FOR_BUILD="gcc"
+  - export CC="$host-gcc"
+  - ./.bootstrap
+  - ./configure --disable-static --enable-fat --disable-documentation --build=$build --host=$host
+  - make -j$(nproc)
+  - make -j$(nproc) check
+  tags:
+  - shared
+  except:
+  - tags
+Debian.cross.arm-linux-gnueabihf:
+  <<: *Debian_cross_template
+Debian.cross.mips-linux-gnu:
+  <<: *Debian_cross_template
+Debian.cross.aarch64-linux-gnu:
+  <<: *Debian_cross_template