ELF: Allow -e <entry> with -shared

It is reasonable to specify an entry point for shared objects - for
example, for the FreeBSD rtld ld-elf.so.1.

Unlike GNU ld we leave the entry address as 0 if -shared is specified
without -e.

Differential Revision:	http://reviews.llvm.org/D15454

llvm-svn: 255349