[libunwind] Avoid using C++ headers.

This is useful for building libunwind on libcxx-free systems.

llvm-svn: 321440
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 91e49066e3067bcc585c434490ba73aef1c3988e
1 file changed