riscv: kprobes: convert one final __ASSEMBLY__ to __ASSEMBLER__

Per the reasoning in commit f811f585 ("riscv: Replace __ASSEMBLY__
with __ASSEMBLER__ in non-uapi headers"), convert one last remaining
instance of __ASSEMBLY__ in the arch/riscv kprobes code.  This entered
the tree from patches that were sent before Thomas' changes; and when
I reviewed the kprobes patches before queuing them, I missed this
instance.

Cc: Nam Cao <namcao@linutronix.dev>
Cc: Thomas Huth <thuth@redhat.com>
Link: https://lore.kernel.org/linux-riscv/16b74b63-f223-4f0b-b6e5-31cea5e620b4@redhat.com/
Link: https://lore.kernel.org/linux-riscv/20250606070952.498274-1-thuth@redhat.com/


Signed-off-by: Paul Walmsley <pjw@kernel.org>