When we see a .arch or .cpu directive, we should try to avoid switching
ARM/Thumb mode if possible.
If we do have to switch modes, we also need to emit the correct mapping
symbol for the new ISA. We did not do this previously, so could emit
ARM code with Thumb mapping symbols (or vice-versa).
The GAS behaviour is to always stay in the same mode, and to emit an
error on any instructions seen when the current mode is not available on
the current target. We can't represent that situation easily (we assume
that Thumb mode is available if ModeThumb is set), so we differ from the
GAS behaviour when switching to a target that can't support the old
mode. I've added a warning for when this implicit mode-switch occurs.
Differential Revision: http://reviews.llvm.org/D18955
llvm-svn: 265936
Oliver Stannard