[gdb] Fix maint selftest -v print_one_insn

[gdb] Fix maint selftest -v print_one_insn

When running the print_one_insn selftests with -v, I get:
...
$ gdb -q -batch -ex "maint selftest -v print_one_insn"
Running selftest print_one_insn::A6.
.shor   0x783eRunning selftest print_one_insn::A7.
trap_s  0x1Running selftest print_one_insn::ARC600.
.shor   0x783eRunning selftest print_one_insn::ARC601.
Running selftest print_one_insn::ARC700.
trap_s  0x1Running selftest print_one_insn::ARCv2.
trap_s  0x1Running selftest print_one_insn::EM.
trap_s  0x1Running selftest print_one_insn::HS.
trap_s  0x1Running selftest print_one_insn::Loongarch32.
...

The insn is written to gdb_stdout, and there is code in the selftest to add a
newline after the insn, which writes to stream().

The stream() ui_file points into a string buffer, which the disassembler uses
before writing to gdb_stdout, so writing into it after the disassembler has
finished has no effect.

Fix this by using gdb_stdlog and debug_printf (which is what the unit test
infrastructure itself uses) instead, such that we have:
...
Running selftest print_one_insn::A6.
.shor   0x783e
Running selftest print_one_insn::A7.
trap_s  0x1
Running selftest print_one_insn::ARC600.
.shor   0x783e
Running selftest print_one_insn::ARC601.
Running selftest print_one_insn::ARC700.
trap_s  0x1
Running selftest print_one_insn::ARCv2.
trap_s  0x1
Running selftest print_one_insn::Loongarch32.
...

Note: I've also removed the printing of arch_name, which would give
us otherwise the redundant:
...
Running selftest print_one_insn::A6.
arc .shor       0x783e
Running selftest print_one_insn::A7.
arc trap_s      0x1
...

Tested on x86_64-linux.