isalbuild: fixed make clean issue.

Symbol-link about isa-l, usually rm -f is enough, but some case it changed. And report errors:
Clean up report error:
rm: cannot remove 'spdk/isa-l/isa-l': Is a directory.
or
rm: cannot remove '/home/vagrant/master/build-20190415032826/spdk/isa-l/isa-l': Is a directory
make[1]: *** [Makefile:53: clean] Error 1
make: *** [/home/vagrant/master/build-20190415032826/spdk/mk/spdk.subdirs.mk:44: isalbuild] Error 2

Change-Id: I7f64e2e9708392c8821e2699fdae26223f472689
Signed-off-by: yidong0635 <dongx.yi@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/448967


Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>