gdb/riscv: convert riscv debug settings to new debug print scheme

gdb/riscv: convert riscv debug settings to new debug print scheme

Convert the RISC-V specific debug settings to use the new debug
printing scheme.  This updates the following settings:

  set/show debug riscv breakpoints
  set/show debug riscv gdbarch
  set/show debug riscv infcall
  set/show debug riscv unwinder

All of these settings now take a boolean rather than an integer, and
all print their output using the new debug scheme.

There should be no visible change for anyone not turning on debug.