Fix two bugs in __cxa_end_cleanup()

Per the EHABI32 specification, __cxa_end_cleanup must take care to
preserve registers before calling _Unwind_Resume().  So, libcxxrt uses
an assembly stub which preserves caller-saved registers around the call
to __cxa_get_cleanup().  But:
- it failed to restore them properly,
- it did not preserve the link register.

Fix both of these problems.  This is needed to fix exception unwinding
on FreeBSD with LLVM 14.  Note that r4 is callee-saved but is pushed
onto the stack to preserve stack pointer alignment.

Sponsored-by: The FreeBSD Foundation