aarch64 multi-arch support (part 2): siginfo fixup

aarch64 multi-arch support (part 2): siginfo fixup

This patch is to fixup the siginfo_t when aarch64 gdb or gdbserver
read from or write to the arm inferior.  It is to convert the
"struct siginfo_t" between aarch64 and arm, which is quite mechanical.

gdb/gdbserver:

2015-09-15  Yao Qi  <yao.qi@linaro.org>

	* linux-aarch64-low.c (aarch64_linux_siginfo_fixup): New
	function.
	(struct linux_target_ops the_low_target): Install
	aarch64_linux_siginfo_fixup.

gdb:

2015-09-15  Yao Qi  <yao.qi@linaro.org>

	* aarch64-linux-nat.c (aarch64_linux_siginfo_fixup): New function.
	(_initialize_aarch64_linux_nat): Call linux_nat_set_siginfo_fixup.
	* nat/aarch64-linux.c (aarch64_compat_siginfo_from_siginfo):
	New function.
	(aarch64_siginfo_from_compat_siginfo): New function.
	* nat/aarch64-linux.h: Include signal.h.
	(compat_int_t, compat_uptr_t, compat_time_t): Typedef.
	(compat_timer_t, compat_clock_t): Likewise.
	(struct compat_timeval): New.
	(union compat_sigval): New.
	(struct compat_siginfo): New.
	(cpt_si_pid, cpt_si_uid, cpt_si_timerid): New macros.
	(cpt_si_overrun, cpt_si_status, cpt_si_utime): Likewise.
	(cpt_si_stime, cpt_si_ptr, cpt_si_addr): Likewise.
	(cpt_si_band, cpt_si_fd): Likewise.