[libunwind] fix dynamic .eh_frame registration (llvm#77185)

Fix this issue
[llvm#76957](llvm#76957)
Libgcc provides __register_frame to register a dynamic .eh_frame
section, while __unw_add_dynamic_eh_frame_section can be used to do the
same in libunwind. However, the address after dynamic .eh_frame are
padding with 0 value, it will be identified as
legal CIE. And __unw_add_dynamic_eh_frame_section will continue to parse
subsequent addresses until illegal memory or other sections are
accessed.
This patch adds length formal parameter for dynamic registration.

libunwind/src/libunwind.cpp

@@ -324,7 +324,7 @@ void __unw_add_dynamic_eh_frame_section(unw_word_t eh_frame_start) {
   CFI_Parser<LocalAddressSpace>::CIE_Info cieInfo;
   CFI_Parser<LocalAddressSpace>::FDE_Info fdeInfo;
   auto p = (LocalAddressSpace::pint_t)eh_frame_start;
-  while (true) {
+  while (LocalAddressSpace::sThisAddressSpace.get32(p)) {
     if (CFI_Parser<LocalAddressSpace>::decodeFDE(
             LocalAddressSpace::sThisAddressSpace, p, &fdeInfo, &cieInfo,
             true) == NULL) {