Make `make clean` output cleaner

[syncom]

Use rm -f to suppress errors on removal of nonexistent files.

Also make clean a phony target.

[syncom]

I'm not sure if it's intentional to show these errors in output when files to remove do not exist, but the "after the change" make clean output seems cleaner.

On a cleanly cloned repo:

Before the change:

make clean
rm gossa
rm: cannot remove 'gossa': No such file or directory
Makefile:68: recipe for target 'clean' failed
make: [clean] Error 1 (ignored)
rm gossa-linux64
rm: cannot remove 'gossa-linux64': No such file or directory
Makefile:68: recipe for target 'clean' failed
make: [clean] Error 1 (ignored)
rm gossa-linux-arm
rm: cannot remove 'gossa-linux-arm': No such file or directory
Makefile:68: recipe for target 'clean' failed
make: [clean] Error 1 (ignored)
rm gossa-linux-arm64
rm: cannot remove 'gossa-linux-arm64': No such file or directory
Makefile:68: recipe for target 'clean' failed
make: [clean] Error 1 (ignored)
rm gossa-mac
rm: cannot remove 'gossa-mac': No such file or directory
Makefile:68: recipe for target 'clean' failed
make: [clean] Error 1 (ignored)
rm gossa-windows.exe
rm: cannot remove 'gossa-windows.exe': No such file or directory
Makefile:68: recipe for target 'clean' failed
make: [clean] Error 1 (ignored)

After the change

$ make clean
rm -f gossa
rm -f gossa-linux64
rm -f gossa-linux-arm
rm -f gossa-linux-arm64
rm -f gossa-mac
rm -f gossa-windows.exe

[pldubouilh]

thank you for this PR ! :)