Add a new gdbarch method to fetch signal information from core files.

Previously the core_xfer_partial method used core_get_siginfo to handle
TARGET_OBJECT_SIGNAL_INFO requests.  However, core_get_siginfo looked for
Linux-specific sections in the core file.  To support fetching siginfo
from cores on other systems, add a new gdbarch method (`core_xfer_siginfo`)
and move the body of the existing core_get_siginfo into a
linux_core_xfer_siginfo implementation of this method in linux-tdep.c.

gdb/ChangeLog:

	* corelow.c (get_core_siginfo): Remove.
	(core_xfer_partial): Use the gdbarch "core_xfer_siginfo" method
	instead of get_core_siginfo.
	* gdbarch.sh (core_xfer_siginfo): New gdbarch callback.
	* gdbarch.h: Re-generate.
	* gdbarch.c: Re-generate.
	* linux-tdep.c (linux_core_xfer_siginfo): New.
	(linux_init_abi): Install gdbarch "core_xfer_siginfo" method.