[ELF] Make -init/-fini options compatible with the gnu linker

The LLD linker searches initializer and finalizer function names
and emits DT_INIT/DT_FINI dynamic table tags to point to these symbols.
The -init/-fini command line options override initializer ("_init") and
finalizer ("_fini") function names used by default.

Now the -init/-fini options do not affect .init_array/.fini_array
sections. The corresponding code has been removed.

Differential Revision: http://reviews.llvm.org/D6578