4.3.9 Z80 banked calls

Banked calls are supported via __banked. Banked calls are done via a trampoline (__sdcc_bcall if --legacy-banking is specified, __sdcc_bcall_abc for z88dk_fastcall, __sdcc_bcall_ehl for other calls). Default trampolines are provided in the library. The default trampolines calls user supplied helper functions set_bank and get_bank that set the current bank to the value in register a, or return the current bank in register a.

For banked functions, the calling convention is slightly different: the stack is always cleared up by the caller. Unless __z88dk_fastcall is used, all parameters are passed on the stack.