Call set_gdbarch_get_siginfo_type in linux_init_abi

Call set_gdbarch_get_siginfo_type in linux_init_abi

linux_get_siginfo_type is installed to many linux gdbarch.  This patch
is to move this to a common area linux-tdep.c:linux_init_abi, so that
linux_get_siginfo_type is installed to every linux gdbarch.  If some
linux gdbarch needs its own version, please override it in
$ARCH_linux_init_abi.  In the testsuite, we enable siginfo related
tests for all linux targets.

gdb:

2015-06-24  Yao Qi  <yao.qi@linaro.org>

	* aarch64-linux-tdep.c (aarch64_linux_init_abi): Don't call
	set_gdbarch_get_siginfo_type.
	* amd64-linux-tdep.c (amd64_linux_init_abi_common): Likewise.
	* arm-linux-tdep.c (arm_linux_init_abi): Likewise.
	* i386-linux-tdep.c (i386_linux_init_abi): Likewise.
	* m68klinux-tdep.c (m68k_linux_init_abi): Likewise.
	* ppc-linux-tdep.c (ppc_linux_init_abi): Likewise.
	* s390-linux-tdep.c (s390_gdbarch_init): Likewise.
	* tilegx-linux-tdep.c (tilegx_linux_init_abi): Likewise.
	* linux-tdep.c (linux_get_siginfo_type): Change it to static.
	(linux_init_abi): Call set_gdbarch_get_siginfo_type.
	* linux-tdep.h (linux_get_siginfo_type): Remove the declaration.

gdb/testsuite:

2015-06-24  Yao Qi  <yao.qi@linaro.org>

	* lib/gdb.exp (supports_get_siginfo_type): Return 1 for all
	linux targets.