riscv: for C functions called only from assembly, mark with __visible

Rather than adding prototypes for C functions called only by assembly
code, mark them as __visible.  This avoids adding prototypes that will
never be used by the callers.  Resolves the following sparse warnings:

arch/riscv/kernel/irq.c:27:29: warning: symbol 'do_IRQ' was not declared. Should it be static?
arch/riscv/kernel/ptrace.c:151:6: warning: symbol 'do_syscall_trace_enter' was not declared. Should it be static?
arch/riscv/kernel/ptrace.c:165:6: warning: symbol 'do_syscall_trace_exit' was not declared. Should it be static?
arch/riscv/kernel/signal.c:295:17: warning: symbol 'do_notify_resume' was not declared. Should it be static?
arch/riscv/kernel/traps.c:92:1: warning: symbol 'do_trap_unknown' was not declared. Should it be static?
arch/riscv/kernel/traps.c:94:1: warning: symbol 'do_trap_insn_misaligned' was not declared. Should it be static?
arch/riscv/kernel/traps.c:96:1: warning: symbol 'do_trap_insn_fault' was not declared. Should it be static?
arch/riscv/kernel/traps.c:98:1: warning: symbol 'do_trap_insn_illegal' was not declared. Should it be static?
arch/riscv/kernel/traps.c:100:1: warning: symbol 'do_trap_load_misaligned' was not declared. Should it be static?
arch/riscv/kernel/traps.c:102:1: warning: symbol 'do_trap_load_fault' was not declared. Should it be static?
arch/riscv/kernel/traps.c:104:1: warning: symbol 'do_trap_store_misaligned' was not declared. Should it be static?
arch/riscv/kernel/traps.c:106:1: warning: symbol 'do_trap_store_fault' was not declared. Should it be static?
arch/riscv/kernel/traps.c:108:1: warning: symbol 'do_trap_ecall_u' was not declared. Should it be static?
arch/riscv/kernel/traps.c:110:1: warning: symbol 'do_trap_ecall_s' was not declared. Should it be static?
arch/riscv/kernel/traps.c:112:1: warning: symbol 'do_trap_ecall_m' was not declared. Should it be static?
arch/riscv/kernel/traps.c:124:17: warning: symbol 'do_trap_break' was not declared. Should it be static?
arch/riscv/kernel/smpboot.c:136:24: warning: symbol 'smp_callin' was not declared. Should it be static?

Based on a suggestion from Luc Van Oostenryck.

This version includes changes based on feedback from Christoph Hellwig
<hch@lst.de>.

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de> # for do_syscall_trace_*