Enable -funwind-tables flag when building libunwind
Or, rather, don't accidentally forget to pass it.
TL;DR: when building libunwind for ARM Linux, we need libunwind to be built with the -funwind-tables flag, because, well ARM EHABI needs unwind info produced by this flag. Without the flag all the procedures in libunwind are marked .cantunwind, which causes all sorts of bad things. From _Unwind_Backtrace not working, to C++ exceptions not being caught (which is the aforementioned bug is about).
Previously, this flag was not added because the CMake check add_compile_flags_if_supported(-funwind-tables) produced a false negative. Why? With this flag, the compiler generates calls to the __aeabi_unwind_cpp_pr0 symbol, which is defined in libunwind itself and obviously is not available at configure time, before libunwind is built. This led to failure at link time during the CMake check. We handle this by disabling the linker for CMake checks in linbunwind.
Reviewers: peter.smith, phosek, EricWF, compnerd, jroelofs, saugustine, miyuki, jfb
Subscribers: mgorny, kristof.beyls, christof, libcxx-commits, miyuki
Differential Revision: https://reviews.llvm.org/D70815