Regression(203b6f31bb71c): libunwind can no longer be built by a bootstrap compiler

[nico]

See repro.tar.gz

At 203b6f3 or later:

% ~/src/llvm-project/out/gn/bin/ld64.lld @response.txt
ld64.lld: error: undefined symbol: __gxx_personality_v0
>>> referenced by Users/thakis/src/llvm-project/out/gn/stage2_unix/obj/libunwind/src/unwind_shared.libunwind.o:(__compact_unwind+0x590)

(Also happens with ld64.)

[nico]

(This is with a GN build. It probably also triggers in a cmake build, but I haven't tested, since setting up a bootstrap libunwind build is more difficult there.)

[nico]

Left some analysis here: https://reviews.llvm.org/D143637#4204277