ARM IAS: support .thumb_set

This performs the equivalent of a .set directive in that it creates a symbol
which is an alias for another symbol or value which may possibly be yet
undefined.  This directive also has the added property in that it marks the
aliased symbol as being a thumb function entry point, in the same way that the
.thumb_func directive does.

The current implementation fails one test due to an unrelated issue.  Functions
within .thumb sections are not marked as thumb_func.  The result is that
the aliasee function is not valued correctly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204059 91177308-0d34-0410-b5e6-96231b3b80d8
MC: change runtime check to an assertion

It is unclear how it would be possible to get M to be NULL in normal scenarios.
Change this to an assert rather than a runtime check as per dblakie's
suggestion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204060 91177308-0d34-0410-b5e6-96231b3b80d8