[clang test] Do not assume default target

Summary:
clang test Driver/darwin-opt-record.c assumes the default target is
x86_64 by its uses of the -arch x86_64 and -arch x86_64h and thus fail
on systems where it is not the case. Adding a target
x86_64-apple-darwin10 reveals another problem: the driver refuses 2
-arch for an assembly output so this commit also changes the output to
be an object file.

Reviewers: thegameg

Reviewed By: thegameg

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70748