[AIX] Remove --as-needed passing into aix linker

Summary:
AIX does not support --as-needed linker options. Remove that option from
aix linker when -lunwind is needed.
For unwinder library, nothing special is needed because by default aix
linker has the as-needed effect for library that's an archive (which is
the case for libunwind on AIX).

Reviewed By: daltenty

Differential Revision: https://reviews.llvm.org/D104314