arm64/fpsimd: Add task_smstop_sm()

In a few places we want to transition a task from streaming mode to
non-streaming mode, e.g. signal delivery where we historically tried to
use an SMSTOP SM instruction.

Add a new helper to manipulate a task's state in the same way as an
SMSTOP SM instruction. I have not added a corresponding helper to
simulate the effects of SMSTART SM. Only ptrace transitions a task into
streaming mode, and ptrace has distinct semantics for such transitions.

Per ARM DDI 0487 L.a, section B1.4.6:

| RRSWFQ
| When the Effective value of PSTATE.SM is changed by any method from 0
| to 1, an entry to Streaming SVE mode is performed, and all implemented
| bits of Streaming SVE register state are set to zero.

| RKFRQZ
| When the Effective value of PSTATE.SM is changed by any method from 1
| to 0, an exit from Streaming SVE mode is performed, and in the
| newly-entered mode, all implemented bits of the SVE scalable vector
| registers, SVE predicate registers, and FFR, are set to zero.

Per ARM DDI 0487 L.a, section C5.2.9:

| On entry to or exit from Streaming SVE mode, FPMR is set to 0

Per ARM DDI 0487 L.a, section C5.2.10:

| On entry to or exit from Streaming SVE mode, FPSR.{IOC, DZC, OFC, UFC,
| IXC, IDC, QC} are set to 1 and the remaining bits are set to 0.

This means bits 0, 1, 2, 3, 4, 7, and 27 respectively, i.e. 0x0800009f

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20250508132644.1395904-9-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

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