tcl: specify ln -sf to remove existing destination file

This fixes below build error when executing
"make tcl-dirclean; make tcl-rebuild".

ln -s tclsh8.4 /opt/test/br-x86/buildroot/output/target/usr/bin/tclsh
ln: failed to create symbolic link '/opt/test/br-x86/buildroot/output/target/usr/bin/tclsh': File exists
make: *** [/opt/test/br-x86/buildroot/output/build/tcl-8.4.19/.stamp_target_installed] Error 1

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>