Add GDB tracer support

[stnolting]

• add GDB helper script: neorv32/sw/example/demo_tracer/neorv32_tracer.gdb
  • restart tracer for CPU 0 or CPU 1
  • dump and pretty-print trace log
• updated documentation's tracer section

Using the tracer from GDB:

(gdb) make clean elf
...
(gdb) load
...
(gdb) c
...
(gdb) source neorv32_tracer.gdb
(gdb) tracer_get
[0] SRC: 0x3b0 -> DST: 0x2cc <TRACE_START>
Line 128 of "main.c" starts at address 0x3b0 <main+212> and ends at 0x3b4 <main+216>.
Line 68 of "main.c" starts at address 0x2cc <test_code> and ends at 0x2d0 <test_code+4>.
...