MC: fix text section characteristics for WoA

link.exe requires that the text section has the IMAGE_SCN_MEM_16BIT flag set.
Otherwise, it will treat the function as ARM.  If this occurs, then jumps to the
function will fail, switching from thumb to ARM mode execution.

With this change, it is possible to link using the MSVC linker as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210415 91177308-0d34-0410-b5e6-96231b3b80d8