Remove a FIXME: use the ios_simulator_version_min linker option consistently.

This was previously only used when explicitly requested with a command line
option because it had to work with some old versions of the linker when it
was first introduced. That is ancient history now, and it should be safe to
use the correct option even when using the IPHONEOS_DEPLOYMENT_TARGET
environment variable to specify that the target is the iOS simulator.
Besides updating the test for this, I also added a few more tests for the
iOS linker options.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219527 91177308-0d34-0410-b5e6-96231b3b80d8