syscall: rework subcall decoding on mips o32

syscall: rework subcall decoding on mips o32

Move syscall subcall handling to the same switch statement that
handles ipc and socket subcalls.

* linux/mips/syscallent-o32.h [LINUX_MIPSO32] (SYS_syscall_subcall):
Define.
* syscall.c (decode_mips_subcall): Rename to decode_syscall_subcall,
conditionalize on SYS_syscall_subcall instead of LINUX_MIPSO32.
(syscall_entering_decode) [LINUX_MIPSO32]: Remove.
(syscall_entering_decode) [SYS_syscall_subcall]: Handle SEN_syscall
using decode_syscall_subcall.