machine: host the per-machine custom init hooks in BANK2

machine_custom_init() runs once at boot, but its code and any tables it
uses were compiled into machine.c and so into the common bank. On the
SWTG018AS-V2.1.0 variant, whose init carries a 21-entry LED register
table, that overflows bank 0 by 0x66 bytes and main no longer links for
MACHINE_PCB_SWTG018AS_V2_1_0; any machine whose init grows can hit the
same wall. SDCC segment pragmas apply file-wide, so the hooks move to a
new machine_init.c compiled into BANK2, and the prototype becomes
banked. machine_check also compiles the new file per machine so the
hooks keep CI syntax coverage.

machine.h

@@ -95,6 +95,6 @@ typedef struct machine_runtime
 	uint8_t isN : 1;
 };
 
-void machine_custom_init(void);
+void machine_custom_init(void) __banked;
 
 #endif