Fix 32-bit arm unwinding through signal frames.

gdb was already okay; libgcc and libunwind need a little extra help.

Bug: 17436734

(cherry picked from commit 148dff3ec6114a03acc722ae43990f1b342abad9)

Change-Id: I2cc997017acc57c930284af5264f353656b98c7b
3 files changed