ci: Exclude C++ sample and test builds on non-Linux hosts

This commit updates the CI workflow such that Zephyr C++ samples and
tests are only built on Linux hosts because there is a known issue
causing the following link error using non-Linux toolchains:

  undefined reference to `_impure_ptr'

Revert this when the GitHub issue #760 is fixed.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

Author: stephanosio

.github/workflows/ci.yml

@@ -1588,12 +1588,18 @@ jobs:
         # Generate test list
         TEST_ARGS="
             -T ${ZEPHYR_ROOT}/samples/hello_world
-            -T ${ZEPHYR_ROOT}/samples/cpp/hello_world
             -T ${ZEPHYR_ROOT}/tests/lib/c_lib
-            -T ${ZEPHYR_ROOT}/tests/lib/cpp
             -T ${ZEPHYR_ROOT}/tests/lib/newlib
             "
 
+        # FIXME: Build C++ samples and tests only for Linux hosts because there
+        #        is a known issue causing link failures on non-Linux hosts (see
+        #        GitHub issue #760).
+        if [ "${{ runner.os }}" == "Linux" ]; then
+          TEST_ARGS+="-T ${ZEPHYR_ROOT}/samples/cpp/hello_world "
+          TEST_ARGS+="-T ${ZEPHYR_ROOT}/tests/lib/cpp "
+        fi
+
         # Run tests with twister
         TWISTER="${ZEPHYR_ROOT}/scripts/twister"
         ${TWISTER} -v -N -M --force-color --inline-logs --retry-failed 3 \