gdbstub: Change syscall callback argument to CPUState

Callback implementations were specific to arm and m68k, so can easily
cast to ARMCPU and M68kCPU respectively.

Prepares for changing GDBState::c_cpu to CPUState.

Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/target-m68k/m68k-semi.c b/target-m68k/m68k-semi.c
index 239fadb..94c4983 100644
--- a/target-m68k/m68k-semi.c
+++ b/target-m68k/m68k-semi.c
@@ -161,8 +161,11 @@
 
 static int m68k_semi_is_fseek;
 
-static void m68k_semi_cb(CPUM68KState *env, target_ulong ret, target_ulong err)
+static void m68k_semi_cb(CPUState *cs, target_ulong ret, target_ulong err)
 {
+    M68kCPU *cpu = M68K_CPU(cs);
+    CPUM68KState *env = &cpu->env;
+
     if (m68k_semi_is_fseek) {
         /* FIXME: We've already lost the high bits of the fseek
            return value.  */