RISC-V: Fix two ARI warnings.

RISC-V: Fix two ARI warnings.

> gdb/riscv-tdep.c:1657: code: %ll: Do not use printf(%ll), instead use printf(%s,phex()) to dump a 'long long' value
gdb/riscv-tdep.c:1657:                  "Writing %lld-byte nop instruction to %s: %s\n",
> gdb/riscv-tdep.c:1658: code: long long: Do not use 'long long', instead use LONGEST
gdb/riscv-tdep.c:1658:                  ((unsigned long long) sizeof (nop_insn)),

fprintf_unfiltered doesn't support z (or j for that matter), and fixing that
is a larger patch than I'd like to write, so this does basically what the
ARI warnings recommends.  We don't need the cast as there is a prototype for
plongest.

	* riscv-tdep.c (riscv_push_dummy_code): Change %lld to %s and use
	plongest instead of unsigned long long cast.