base.bbclass: clean up dead symlink when handling hosttools

base.bbclass: clean up dead symlink when handling hosttools

When some hosttool is a dead symlink, bitbake any recipe will
fail with error like below.

  FileExistsError: [Errno 17] File exists: '/usr/bin/chrpath' -> '/path/to/builddir/tmp-glibc/hosttools/chrpath'

So we remove dead symlink under hostools/ directory to avoid
such error.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>