unwind: Fix libc++abi and libgcc build.

To build libc++abi without libunwind, we should make sure that all
function calls to _Unwind_{Get,Set}{GR,IP}() are inlined as function
calls to _Unwind_VRS_{Get,Set}().  Otherwise, libc++abi.so will fail to
link since libgcc does not provide these symbol at all.

This commit fixes the problem by providing both the inlined version and
exported version.

llvm-svn: 243073