gcc/gcc-final: use the common HOST_GCC_CONFIGURE_SYMLINK macro

Each stage of the gcc build needs to make a small dance before the
configuration step to create a build sub-directory and a symbolic link
to the configure script. The common gcc.mk had a
HOST_GCC_CONFIGURE_SYMLINK already used by the gcc-initial and
gcc-intermediate steps, but the gcc-final step wasn't using it.

This commit fixes this inconsistency, and therefore removes the
HOST_GCC_FINAL_CONFIGURE_SYMLINK macro.

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