[libunwind] Fix compilation for the x32 ABI.

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.

Author: alexrp

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