gcc: force symlink creation

The HOST_GCC_CONFIGURE_SYMLINK creates a symbolic link
$(@D)/build/configure -> $(@D)/configure for each build step of
gcc. However, it was only using 'ln -s' and not 'ln -sf', which was
causing problems when doing 'make host-gcc-final-reconfigure' for
example, because the configure commands (including this macro) are
being re-executed, but the symbolic link already exists.

Changing this symbolic link creation to 'ln -sf' fixes this problem.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>