fix(ulibc): add null-pointer guards in __libc_init_array/fini_array

fix(ulibc): add null-pointer guards in __libc_init_array/fini_array

In a PIC shared library build, linker-generated symbols like
__init_array_start, __fini_array_end, and _init may remain
SHN_UNDEF (value 0) when the library has no .init_array/.fini_array
sections. The GOT entries for these resolve to 0.

Added null-pointer checks using volatile pointers (to prevent the
compiler from assuming the address is always non-null) for all
init/fini array pointers and for _init/_fini function pointers.
This prevents crashes when these symbols are unresolved.