[libunwind] Support building hermetic static library

This is useful when the static libunwind library is being linked into
shared libraries that may be used in with other shared libraries that
use different unwinder. We want to avoid avoid exporting libunwind
symbols in those cases. This achieved by a new CMake option which can be
enabled by libunwind vendors as needed.

The same CMake option has already been added to libc++ and libc++abi in
D55404 and D56026.

Differential Revision: https://reviews.llvm.org/D57107

llvm-svn: 352559

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