The 8051 has built in support for bank switching.
The Z80 does not.
SDCC has support for 8051 bank switching.
Z80 bank switching (and for other CPU's that don't natively support it) is usually done with a simple latch circuit.
Why can't SDCC provide Z80 with bank switching support? If I had to do it myself, what files in the Z80 would I need to modify? Which files in 8051 provide the bank switching?