Makefiles: Fix CMAKE_EXPORT_COMPILE_COMMANDS crash with custom compile rule

Extend the fix from commit 67e2130c96 (Makefiles: Fix
CMAKE_EXPORT_COMPILE_COMMANDS crash with custom compile rule,
2020-11-20, v3.19.1~8^2) to work for the `<FLAGS>` placeholder too.
Also fix the test case to actually enable `EXPORT_COMPILE_COMMANDS`
on the test target.

Fixes: #22261