[ARM] In thumb mode, emit directive ".code 16" before file level inline
assembly instructions.

This is necessary to ensure ARM assembler switches to Thumb mode before it
starts assembling the file level inline assembly instructions at the beginning
of a .s file.

<rdar://problem/17757232>



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213924 91177308-0d34-0410-b5e6-96231b3b80d8