MIPS: AT_HWCAP aux vector infrastructure

In order for userland to determine whether various features are safe to
use, it will need to know both that the hardware supports those features
and that the kernel is recent enough & configured appropriately to
support them. For example under the O32 modeless FP proposal the dynamic
linker & ifunc resolvers will need this information.  The kernel is the
only thing in a position to know availability accurately, so the kernel
needs to provide the information to userland. This patch introduces the
infrastructure to provide the AT_HWCAP aux vector to userland in order
to provide that information. It also defines the 2 currently specified
flags, which indicate MIPSr6 & MSA support.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Matthew Fortune <matthew.fortune@imgtec.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Maciej W. Rozycki <macro@linux-mips.org>
Cc: Alex Smith <alex@alex-smith.me.uk>
Cc: linux-kernel@vger.kernel.org
Cc: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Cc: Markos Chandras <markos.chandras@imgtec.com>
Cc: Ingo Molnar <mingo@kernel.org>
Patchwork: https://patchwork.linux-mips.org/patch/10797/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>