Add support for AT_HWCAP2 auxv entry

Recent ppc64 Linux kernels provide a new auxv entry AT_HWCAP2,
which is currently not recognized by GDB, causing every use of
"info auxv" to show an error.

This commit adds the AT_HWCAP2 define to include/elf/common.h
and handles it in GDB.

include/elf/ChangeLog:

	* common.h (AT_HWCAP2): Define.

gdb/ChangeLog:

	* auxv.c (fprint_target_auxv): Handle AT_HWCAP2.