PlatformMacOSX should be activated for lldb built to run on an iOS etc device

In the changes Jonas made in https://reviews.llvm.org/D117340 , a
small oversight was that PlatformMacOSX (despite the name) is active
for any native Darwin operating system, where lldb and the target
process are running on the same system. This patch uses compile-time
checks to return the appropriate OSType for the OS lldb is being
compiled to, so the "host" platform will correctly be selected when
lldb & the inferior are both running on that OS. And a small change
to PlatformMacOSX::GetSupportedArchitectures which adds additional
recognized triples when running on macOS but not other native Darwin
systems.

Differential Revision: https://reviews.llvm.org/D120517
rdar://89247060