arm64/sve: Add prctl controls for userspace vector length management

This patch adds two arm64-specific prctls, to permit userspace to
control its vector length:

 * PR_SVE_SET_VL: set the thread's SVE vector length and vector
   length inheritance mode.

 * PR_SVE_GET_VL: get the same information.

Although these prctls resemble instruction set features in the SVE
architecture, they provide additional control: the vector length
inheritance mode is Linux-specific and nothing to do with the
architecture, and the architecture does not permit EL0 to set its
own vector length directly.  Both can be used in portable tools
without requiring the use of SVE instructions.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Alex Bennée <alex.bennee@linaro.org>

---

**Dropped at v3** Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
due to non-trivial changes/fixes after v2.

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

Requested by Catalin Marinas:

 * Replace static __maybe_unused functions with static inlines.

   (Retaining Catalin's Reviewed-by with his approval.)