OpenBSD support. (apple#291)

On this platform, the TIOCGWINSZ ioctl identifier is a complex macro.
Since we don't have a C bridging header obviously available to get the
flattened value, supply is the flattened value obtained elsewhere. This
is of course brittle but this is the simplest way around this for now.

Additionally, ensure we support the platform architecture name, where
the x86_64 architecture is called amd64 instead.

Sources/ArgumentParser/Usage/HelpGenerator.swift

@@ -325,7 +325,13 @@ func _terminalSize() -> (width: Int, height: Int) {
           height: Int(csbi.srWindow.Bottom - csbi.srWindow.Top) + 1)
 #else
   var w = winsize()
+#if os(OpenBSD)
+  // TIOCGWINSZ is a complex macro, so we need the flattened value.
+  let tiocgwinsz = Int32(0x40087468)
+  let err = ioctl(STDOUT_FILENO, tiocgwinsz, &w)
+#else
   let err = ioctl(STDOUT_FILENO, TIOCGWINSZ, &w)
+#endif
   let width = Int(w.ws_col)
   let height = Int(w.ws_row)
   guard err == 0 else { return (80, 25) }

cmake/modules/SwiftSupport.cmake

@@ -27,6 +27,8 @@ function(get_swift_host_arch result_var_name)
     set("${result_var_name}" "armv7" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "armv7l")
     set("${result_var_name}" "armv7" PARENT_SCOPE)
+  elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "amd64")
+    set("${result_var_name}" "amd64" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "AMD64")
     set("${result_var_name}" "x86_64" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "IA64")