ARM: prevent crash on ELF directives on COFF

Certain directives are unsupported on Windows (some of which could/should be
supported).  We would not diagnose the use but rather crash during the emission
as we try to access the Target Streamer.  Add an assertion to prevent creating a
NULL reference (which is not permitted under C++) as well as a test to ensure
that we can diagnose the disabled directives.

llvm-svn: 218014