Preparation for new siginfo on Linux

Preparation for new siginfo on Linux

First add new structure and function to allow architecture customization
for the siginfo structure.

2016-01-15  Walfred Tedeschi  <walfred.tedeschi@intel.com>

gdb/ChangeLog:

	* linux-tdep.h (linux_siginfo_extra_field_values): New enum values.
	(linux_siginfo_extra_fields): New enum type.
	* linux-tdep.c (linux_get_siginfo_type_with_fields): New function.
	(linux_get_siginfo_type): Use new function.