gdbstub: move syscall handling to new file

Our GDB syscall support is the last chunk of code that needs target
specific support so move it to a new file. We take the opportunity to
move the syscall state into its own singleton instance and add in a
few helpers for the main gdbstub to interact with the module.

I also moved the gdb_exit() declaration into syscalls.h as it feels
pretty related and most of the callers of it treat it as such.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

Message-Id: <20230302190846.2593720-22-alex.bennee@linaro.org>
Message-Id: <20230303025805.625589-22-richard.henderson@linaro.org>

semihosting/arm-compat-semi.c

@@ -34,6 +34,7 @@
 #include "qemu/osdep.h"
 #include "qemu/timer.h"
 #include "exec/gdbstub.h"
+#include "gdbstub/syscalls.h"
 #include "semihosting/semihost.h"
 #include "semihosting/console.h"
 #include "semihosting/common-semi.h"

semihosting/guestfd.c

@@ -9,7 +9,7 @@
  */
 
 #include "qemu/osdep.h"
-#include "exec/gdbstub.h"
+#include "gdbstub/syscalls.h"
 #include "semihosting/semihost.h"
 #include "semihosting/guestfd.h"
 #ifdef CONFIG_USER_ONLY

semihosting/syscalls.c

@@ -7,8 +7,8 @@
  */
 
 #include "qemu/osdep.h"
-#include "exec/gdbstub.h"
 #include "cpu.h"
+#include "gdbstub/syscalls.h"
 #include "semihosting/guestfd.h"
 #include "semihosting/syscalls.h"
 #include "semihosting/console.h"