Reland "Enable -funwind-tables flag when building libunwind"
The original commit set CMAKE_TRY_COMPILE_TARGET_TYPE to
STATIC_LIBRARY globally in libunwind/CMakeLists.txt, which effectively
disabled the linking step in CMake checks.
This broke some builds (see 938c70b86c7d2165f8c28d5700e9c1ac1263307e).
Here we set CMAKE_TRY_COMPILE_TARGET_TYPE to
STATIC_LIBRARY only when checking for presence of the -funwind-tables
flag, and then set it back to the original value so it doesn't affect
Reviewers: mstorsjo, jfb
Subscribers: mgorny, christof, libcxx-commits
Differential Revision: https://reviews.llvm.org/D71117