[libunwind] Bump to CXX_STANDARD 17 (#75986)

libunwind uses C-style and low-level C++, so the language standard
doesn't matter that much, but bumping to C++17 aligns with the rest of
LLVM and enables some features that would cause pedantic warnings in
C++11 (e.g. -Wc++17-attribute-extensions for [[fallthrough]]/
[[nodiscard]]/[[maybe_unused]]). (Contributors might use these features
unaware of the pedantic warnings).

Suggested-by: Christopher Di Bella <cjdb@google.com>