symtab: add support for CRC values from __kcrctab

symtab: add support for CRC values from __kcrctab

New kernels changed the format of storing CRC values from absolute
symbol value to the address in __kcrctab or __kcrctab_gpl section.
This change adds support for CRC values described in this format.

	* src/abg-elf-helpers.h (get_crc_for_symbol): Defined new
	helper function to extract CRC from ELF symbol.
	* src/abg-elf-helpers.cc (get_crc_for_symbol): Implemented this
	function with support of old and new CRC values format.
	* src/abg-symtab-reader.cc (symtab::load_): Used the new
	function when building CRC values map.

Change-Id: I7de5c737d5caaef0c5b7b2ea0d448368889a16be
Signed-off-by: Aleksei Vetrov <vvvvvv@google.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>