[libunwind] Fix compilation for the x32 ABI. (#116608)

This would previously fail the static assertions in `UnwindCursor.hpp`
due to `UnwindCursor`'s size not matching `unw_cursor_t`'s size. As is
done for MIPS N32, this just declares the appropriate size in
`__libunwind_config.h`.

libunwind/include/__libunwind_config.h

@@ -53,6 +53,9 @@
 #    else
 #      define _LIBUNWIND_CURSOR_SIZE 66
 #    endif
+#  elif defined(__ILP32__)
+#    define _LIBUNWIND_CONTEXT_SIZE 21
+#    define _LIBUNWIND_CURSOR_SIZE 28
 #  else
 #    define _LIBUNWIND_CONTEXT_SIZE 21
 #    define _LIBUNWIND_CURSOR_SIZE 33