change --gc-section flag to --gc-sections flag

[Octavian-Zhang]

There's a typo in the example CMakeLists.txt files, see from the GCC manual there's no --gc-section option, the correct one should be --gc-sections: https://gcc.gnu.org/onlinedocs/gnat_ugn/Compilation-options.html

[richiprosima]

Build status:

• Linux
• Windows

[pablogs9]

@Octavian-Zhang please fix DCO

@Acuadros95 PTAL and S&M