[Driver] Respect -target when determining target platform on Apple

platforms.

Set the target OS based on -target if it is present on the command
line and -arch is not.

With this commit, "-target x86_64-apple-ios8.0" does the same thing as
"-arch x86_64 -mios-version-min=8.0".

rdar://problem/21012522

llvm-svn: 307982