This uses the new layout of the upstream repository, which was recently
migrated to GitHub, and converted into a "monorepo". That is, most of
the earlier separate sub-projects with their own branches and tags were
consolidated into one top-level directory, and are now branched and
tagged together.
Updating the vendor area to match this layout is next.
Drop portions that are unlit or redundant with llvm-libunwind: builtin.c,
unwind.h, and unwind_arm_ehabi_stub.c.
This code should now work with -fPIE binaries, should we choose to build any
that way.
When backtrace() array is full, signal an error so the underlying
Itanium-style C++ exception handling library (llvm-libunwind) knows to stop
tracing instead of continuing. (It should stop on its own when it finishes
unwinding, so this is mostly an extra seatbelt against an infinite loop bug
in the unwinder.)
