Introduce HAVE_STRUCT_TCB_EXT_ARG macro

Introduce HAVE_STRUCT_TCB_EXT_ARG macro

Check for "if HAVE_STRUCT_TCB_EXT_ARG" instead of
"if defined LINUX_MIPSN32 || defined X32".

* defs.h (HAVE_STRUCT_TCB_EXT_ARG): Define for LINUX_MIPSN32 || X32.
(struct tcb): Check it instead of LINUX_MIPSN32 || X32.
(RVAL_LUDECIMAL): Likewise.
* io.c (print_lld_from_low_high_val): Likewise.
* lseek.c (SYS_FUNC(lseek)): Likewise.
* mem.c (SYS_FUNC(mmap)): Likewise.
* syscall.c (trace_syscall_exiting): Likewise.
* util.c (getllval): Likewise.