gdb/riscv: Fixes to printf format strings

gdb/riscv: Fixes to printf format strings

Some of the format strings used in the new riscv target were incorrect,
resulting in build failures on some hosts.  This commit does the
following:

  1. Uses core_addr_to_string for formatting CORE_ADDR types.
  2. Fixes legacy use of stderr for logging in one place that got
     missed, instead gdb_stdlog is used.
  3. Re-indent a few printf related lines that were wrong.

This should resolve some (but not all) of the build failures the new
riscv target introduced.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_print_arg_location): Add header comment,
	change parameter type.  Use GDB's print functions, and use
	core_addr_to_string where appropriate.
	(riscv_push_dummy_call): Use core_addr_to_string where
	appropriate, update call to riscv_print_arg_location, and reindent
	a few lines.
	(riscv_return_value): Update call to riscv_print_arg_location.