Reland "Enable `-funwind-tables` flag when building libunwind"

Summary:
Relands https://reviews.llvm.org/D70815.

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 938c70b8).

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
other checks.

Reviewers: mstorsjo, jfb

Subscribers: mgorny, christof, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D71117