[ELF][MC] Set types of aliases of IFunc to STT_GNU_IFUNC

```
.type  foo,@gnu_indirect_function
.set   foo,foo_resolver

.set foo2,foo
.set foo3,foo2
```

The types of foo2 and foo3 should be STT_GNU_IFUNC, but we currently
resolve them to the type of foo_resolver. This patch fixes it.

Differential Revision: https://reviews.llvm.org/D67206
Patch by Senran Zhang

llvm-svn: 371312