This commit pulls out the functionality necessary to implement stack
unwinding in rtld into macros and implements longjmp in terms of them.
Using these macros, this commit implements the functionality necessary
to support exception handling for libunwind. Additionally, it adds a new
otype which is reserved for the unwinding library to use.

Instead, use the get_trusted_frame macro to obtain the trusted frame in
C.

This commit also fixes the missing unw_getcontext_unsealed in trusted
symbols and moves libunwind symbols closer to the setjmp/longjmp ones.