arm64/sve: Detect SVE and activate runtime support

This patch enables detection of hardware SVE support via the
cpufeatures framework, and reports its presence to the kernel and
userspace via the new ARM64_SVE cpucap and HWCAP_SVE hwcap
respectively.

Userspace can also detect SVE using ID_AA64PFR0_EL1, using the
cpufeatures MRS emulation.

When running on hardware that supports SVE, this enables runtime
kernel support for SVE, and allows user tasks to execute SVE
instructions and make of the of the SVE-specific user/kernel
interface extensions implemented by this series.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>

---

**Dropped** Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
The entry.S changes need reviewing; however the rest of the cpufeatures
support (reviewed by Suzuki) has not changed and shouldn't need re-
review.

Changes since v3
----------------

Changes requested by Catalin Marinas / Will Deacon:

 * ABI change: Zero SVE regs on syscall entry:

   Move to asm alternatives for skipping SVE discard at syscall entry,
   now that we have a suitable cpucap defined.

Miscellaneous:

 * Add HWCAP_SVE description to elf_hwcaps.txt