Update running bionic unit tests on host.

Add a bionic-unit-tests-glibc-run target to run the glibc bionic unit tests.

Modify the bionic-unit-tests-run-on-host to make sure that the /system/bin
directory is created properly.

Also remove the EXTERNAL_STORAGE variable which isn't used any more.

Bug: 11234772
Change-Id: I9aea501d05700b29e938f672474d550b1872a78b