Makefile: Remove coverage files on 'make clean'

This patch updates a variety of Makefiles in Asterisk's build system to
remove .gcda and .gcno files when 'make clean' is executed. These files
are generated when '--enable-coverage' is passed to the Asterisk
configure script.

Change-Id: Ib70b41eea2ee2908885bff02e80faf9f40c84602