[ELF2] Add DT_INIT and DT_FINI dynamic table entries

The entries are added if there are "_init" or "_fini" entries in
the symbol table respectively. According to the behavior of ld,
entries are inserted even for undefined symbols.

Symbol names can be overridden by using -init and -fini command
line switches. If used, these switches neither add new symbol table
entries nor require those symbols to be resolved.

Differential Revision: http://reviews.llvm.org/D13385

llvm-svn: 249297