Honor -fuse-init-array when os is not specified on x86

Currently -fuse-init-array option is not effective when target triple
does not specify os, on x86,x86_64.
i.e.

// -fuse-init-array is not honored.
$ clang -target i386 -fuse-init-array test.c -S

// -fuse-init-array is honored.
$ clang -target i386-linux -fuse-init-array test.c -S

This patch fixes first case.
And does cleanup.

Reviewers: rnk, craig.topper, fhahn, echristo

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D71360