[ARM] Remove field syscall_next_pc in struct gdbarch_tdep

[ARM] Remove field syscall_next_pc in struct gdbarch_tdep

Field syscall_next_pc in struct gdbarch_tdep was to calculate the
next pc of syscall instruction.  On linux target, syscall_next_pc
is set to arm_linux_syscall_next_pc, to do linux specific things.
However, after we have struct arm_get_next_pcs_ops, we can do the
same thing in struct arm_get_next_pcs_ops field syscall_next_pc,
so syscall_next_pc in struct gdbarch_tdep is not needed any more.

gdb:

2016-01-14  Yao Qi  <yao.qi@linaro.org>

	* arm-linux-tdep.c (arm_linux_get_next_pcs_syscall_next_pc):
	Declare.
	(arm_linux_get_next_pcs_ops): Install
	arm_linux_get_next_pcs_syscall_next_pc.
	(arm_linux_syscall_next_pc): Change to ...
	(arm_linux_get_next_pcs_syscall_next_pc): ... it.
	(arm_linux_init_abi): Don't set tdep->syscall_next_pc.
	* arm-tdep.c (arm_get_next_pcs_syscall_next_pc): Declare.
	(arm_get_next_pcs_syscall_next_pc): Make it static.  Don't
	call tdep->syscall_next_pc.
	* arm-tdep.h (struct gdbarch_tdep) <syscall_next_pc>: Remove.
	(arm_get_next_pcs_syscall_next_pc): Remove.