riscv: support PREEMPT_DYNAMIC with static keys

Currently, each architecture can support PREEMPT_DYNAMIC through
either static calls or static keys. To support PREEMPT_DYNAMIC on
riscv, we face three choices:

1. only add static calls support to riscv
As Mark pointed out in commit 99cf983c ("sched/preempt: Add
PREEMPT_DYNAMIC using static keys"), static keys "...should have
slightly lower overhead than non-inline static calls, as this
effectively inlines each trampoline into the start of its callee. This
may avoid redundant work, and may integrate better with CFI schemes."
So even we add static calls(without inline static calls) to riscv,
static keys is still a better choice.

2. add static calls and inline static calls to riscv
Per my understanding, inline static calls requires objtool support
which is not easy.

3. use static keys

While riscv doesn't have static calls support, it supports static keys
perfectly. So this patch selects HAVE_PREEMPT_DYNAMIC_KEY to enable
support for PREEMPT_DYNAMIC on riscv, so that the preemption model can
be chosen at boot time. It also patches asm-generic/preempt.h, mainly
to add __preempt_schedule() and __preempt_schedule_notrace() macros
for PREEMPT_DYNAMIC case. Other architectures which use generic
preempt.h can also benefit from this patch by simply selecting
HAVE_PREEMPT_DYNAMIC_KEY to enable PREEMPT_DYNAMIC if they supports
static keys.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230716164925.1858-1-jszhang@kernel.org


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>