[ARM] Add ".code 32" to functions in the ARM instruction set

Before, only Thumb functions were marked as ".code 16". These
".code x" directives are effective until the next directive of its
kind is encountered. Therefore, in code with interleaved ARM and
Thumb functions, it was possible to declare a function as ARM and
end up with a Thumb function after assembly. A test has been added.

An existing test has also been fixed to take this change into
account.

Reviewers: aschwaighofer, t.p.northover, jmolloy, rengolin

Subscribers: aemerson, rengolin, llvm-commits

Differential Revision: https://reviews.llvm.org/D24337

llvm-svn: 281324