Import LLVM libunwind snapshot revision 272680

Significant upstream revisions:

260595: [AArch64] Fix libunwind build when using GNU assembler
270692: Introduce a native-only unwinder build.
270972: Disable cross-unwinding by default.
271004: [libunwind] Improve unwinder stack usage - II
272680: [libunwind] Improve unwinder stack usage - III

Obtained from: https://llvm.org/svn/llvm-project/libunwind/trunk/