Fix ARMv4T (Thumb1) epilogue generation

Summary:
Before ARMv5T, Thumb1 code could not pop PC, as described at D14357 and D14986;
so we need the special fixup in the epilogue.

Reviewers: jroelofs, qcolombet

Subscribers: aemerson, llvm-commits, rengolin

Differential Revision: http://reviews.llvm.org/D15126

llvm-svn: 255047