cmake: introduce a target coverage-reset

The path introduce a CMake target `coverage-reset` that resets code
coverage counters:

```
$ make coverage-report
Building coverage report
(INFO) - MainThread - Reading coverage data...
(INFO) - MainThread - Writing coverage report...
lines: 53.2% (5949 out of 11177)
functions: 60.0% (631 out of 1051)
branches: 36.2% (3300 out of 9106)
decisions: 47.6% (1486 out of 3119)
Built target coverage-report

$ make coverage-reset
[100%] Reset code coverage counters
[100%] Built target coverage-reset

$ make coverage-report
Building coverage report
(INFO) - MainThread - Reading coverage data...
(INFO) - MainThread - Writing coverage report...
lines: 0.0% (0 out of 11177)
functions: 0.0% (0 out of 1051)
branches: 0.0% (0 out of 9106)
decisions: 0.0% (0 out of 3119)
Built target coverage-report
$
```

Note, files 'src/host/*.gcda' generated in LuaJIT directory are not
removed, because CMake cannot remove recursively by globbing.
Files 'src/host/*.gcda' are not used for building coverage report,
so it is ok.

cmake/CodeCoverage.cmake

@@ -52,5 +52,20 @@ add_custom_command(TARGET ${target_name}
   WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
 )
 
+# The .gcda count data file is generated when a program containing
+# object files built with the GCC -fprofile-arcs option is executed.
+# https://gcc.gnu.org/onlinedocs/gcc/Gcov-Data-Files.html
+set(GCDA_FILES "${LUA_SOURCE_DIR}/*.gcda")
+if(USE_LUAJIT)
+  # Files 'src/host/*.gcda' are not removed, because
+  # CMake cannot remove recursively by globbing.
+  # Files 'src/host/*.gcda' are not used for building coverage report.
+  set(GCDA_FILES "${LUA_SOURCE_DIR}/src/*.gcda")
+endif(USE_LUAJIT)
+add_custom_target(coverage-reset
+  COMMENT "Reset code coverage counters"
+  COMMAND ${CMAKE_COMMAND} -E rm -f ${GCDA_FILES}
+)
+
 message(STATUS "Code coverage HTML report: ${COVERAGE_HTML_REPORT}")
 message(STATUS "Code coverage XML report: ${COVERAGE_XML_REPORT}")