[libunwind] Disable cross-unwinding by default.

Cross unwinding requires larger sizes for unw_context_t and unw_cursor_t
buffers (large enough to hold the VRS of any architecture). This is not
desirable for the most common situation where libunwind is used for native
unwinding only.

This patch makes native unwinding the default build configuration. This
will start testing the tigher (compile-time) bounds of unw_context_t
and unw_cursor_t buffers for each architecture.

Change-Id: Ic61c476a75603ca4812959c233cfe56f83dc0b00
llvm-svn: 270972