[mach-o] add MachOLinkingContext::configure(type, arch, os)

It will configure resonable defaults for other settings in the 
MachOLinkingContext object based on the parameters.  

Patch by Joe Ranieri


git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197851 91177308-0d34-0410-b5e6-96231b3b80d8