arch: consolidate arch_irq_work_raise prototypes

The prototype was hidden in an #ifdef on x86, which causes a warning:

kernel/irq_work.c:72:13: error: no previous prototype for 'arch_irq_work_raise' [-Werror=missing-prototypes]

Some architectures have a working prototype, while others don't.
Fix this by providing it in only one place that is always visible.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

riscv: Fix irq_work when SMP is disabled

irq_work is triggered via an IPI, but the IPI infrastructure is not
included in uniprocessor kernels. As a result, irq_work never runs.
Fall back to the tick-based irq_work implementation on uniprocessor
configurations.

Fixes: 298447928bb1 ("riscv: Support irq_work via self IPIs")
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20220430030025.58405-1-samuel@sholland.org
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

riscv: Support irq_work via self IPIs

Support for arch_irq_work_raise() and arch_irq_work_has_interrupt() was
missing from riscv (a prerequisite for FULL_NOHZ).

Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>