The following discussion is meant to explain the difference between hosts and targets. Host refers to attributes of the system where GDB runs. Target refers to the system where the program being debugged executes. In most cases they are the same machine, in which case a third type of Native attributes come into play. Defines and include files needed to build on the host are host support.
Examples are tty support, system defined types, host byte order, host float format.
Defines and information needed to handle the target format are target dependent. Examples are the stack frame format, instruction set, breakpoint instruction, registers, and how to set up and tear down the stack to call a function. Information that is only needed when the host and target are the same, is native dependent. The various macros needed for finding the registers in the upage , running ptrace , and such are all in the native-dependent files. Core file handling and setjmp handling are two common cases.