Set eRegisterKindEHFrame register numbers for 32 bit ARM register contexts in minidumps

Stack unwinding was sometimes failing when trying to unwind stacks in 32 bit ARM. I discovered this was because the EH frame register numbers were not set. This patch fixes this issue and adds a unit test to verify this doesn't regress.

Differential Revision: https://reviews.llvm.org/D68088

llvm-svn: 374246