[ARM] Fix Thumb text sections' flags under COFF/Windows

The main issue here is that the "thumb" flag wasn't set for some of these
sections, making MSVC's link.exe fails to correctly relocate code
against the symbols inside these sections. link.exe could fail for
instance with the "fixup is not aligned for target 'XX'" error. If
linking doesn't fail, the relocation process goes wrong in the end and
invalid code is generated by the linker.

This patch adds Thumb/ARM information so that the right flags are set
on COFF/Windows.

Patch by Adrien Guinet.

llvm-svn: 273880