Emit word of zeroes after the last instruction as a start of the mandatory

Emit word of zeroes after the last instruction as a start of the mandatory
traceback table on PowerPC64. This helps gdb handle exceptions. The other
mandatory fields are ignored by gdb and harder to implement so just add
there a FIXME.

Patch by Bill Schmidt. PR13641.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162778 91177308-0d34-0410-b5e6-96231b3b80d8