Fix thumbv4t indirect calls

So there are a couple of issues with indirect calls on thumbv4t. First, the most
'obvious' instruction, 'blx' isn't available until v5t. And secondly, the
next-most-obvious sequence: 'mov lr, pc; bx rN' doesn't DTRT in thumb code
because the saved off pc has its thumb bit cleared, so when the callee returns
we end up in ARM mode.... yuck.

The solution is to 'bl' to a nearby landing pad with a 'bx rN' in it.

We could cut down on code size by sharing the landing pads between call sites
that are close enough, but for the moment let's do correctness first and look at
performance later.


Patch by: Iain Sandoe

http://reviews.llvm.org/D6519

llvm-svn: 223380