Merge LLVM libunwind fixes

r302475: libunwind: limit stack usage in unwind cursor

This may be reworked upstream but in the interim should address the
stack usage issue reported in the PR.

r303061: libunwind: Properly align _Unwind_Exception.

_Unwind_Exception is required to be double word aligned. GCC has
interpreted this to mean "use the maximum useful alignment for the
target" so follow that lead.

PR: 206384 (r302475)
Obtained from: LLVM review D22543 (r303061)
Approved by: re (gjb)