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
MFC after: 1 week
cbd1e83154