riscv: asm: use .insn for making custom instructions

riscv: asm: use .insn for making custom instructions

The assembler has .insn for building custom instructions
now, so change the .4byte to .insn. This ensures the output
is marked as an instruction and not as data which may
confuse both debuggers and anything else that relies on
this sort of marking.

Add an ASM_INSN_I() wrapper in asm.h to allow the selecting
of how this is output so older assemblers are still good.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Link: https://lore.kernel.org/r/20251024171640.65232-1-ben.dooks@codethink.co.uk
Signed-off-by: Paul Walmsley <pjw@kernel.org>