Provide a platform-specific linker override

[cmcgee1024]

SwiftPM currently uses the lld-link linker on Windows and would like to preserve that selection when using SwiftBuild to build for Windows. Currently, it checks the host platform and conditionally overrides the ALTERNATE_LINKER build setting to get the desired behaviour. Instead, there should be a way to describe the alternate linker on a per-platform basis, or SwiftBuild could adopt the same linker as the default for Windows.