esp32c3: add support for GDB debugging

[aykevl]

You can now debug the ESP32-C3 from the TinyGo command line, like this:

tinygo flash -target=esp32c3 examples/serial
tinygo gdb -target=esp32c3 examples/serial

It's important to flash before running tinygo gdb, because loading a
new firmware from GDB has not yet been implemented.

Probably the easiest way to connect to the ESP32-C3 is by using the
built-in JTAG connection. See:
https://docs.espressif.com/projects/esp-idf/en/latest/esp32c3/api-guides/jtag-debugging/configure-builtin-jtag.html

You will need to make sure that the openocd command in your $PATH is
the one from Espressif. Otherwise GDB will hang. You can debug this by
supplying the -ocd-output flag:

$ tinygo gdb -target=esp32c3 -ocd-output examples/serial
Open On-Chip Debugger 0.10.0
openocd: Licensed under GNU GPL v2
openocd: For bug reports, read
openocd: 	http://openocd.org/doc/doxygen/bugs.html
openocd: embedded:startup.tcl:60: Error: Can't find interface/esp_usb_jtag.cfg
openocd: in procedure 'script'
openocd: at file "embedded:startup.tcl", line 60

Make sure to configure OpenOCD correctly, until you get the correct
version (that includes the string "esp32"):

$ openocd --version
Open On-Chip Debugger  v0.10.0-esp32-20210721 (2021-07-21-13:33)
Licensed under GNU GPL v2
For bug reports, read
    http://openocd.org/doc/doxygen/bugs.html

If you are on Linux, you may also get the following error:

$ tinygo gdb -target=esp32c3 -ocd-output examples/serial
Open On-Chip Debugger  v0.10.0-esp32-20210721 (2021-07-21-13:33)
openocd: Licensed under GNU GPL v2
openocd: For bug reports, read
openocd: 	http://openocd.org/doc/doxygen/bugs.html
openocd: Info : only one transport option; autoselect 'jtag'
openocd: adapter speed: 40000 kHz
openocd:
openocd: Warn : Transport "jtag" was already selected
openocd: Info : Listening on port 6666 for tcl connections
openocd: Info : Listening on port 4444 for telnet connections
openocd: Error: libusb_open() failed with LIBUSB_ERROR_ACCESS
openocd: Error: esp_usb_jtag: could not find or open device!

The error LIBUSB_ERROR_ACCESS means that there is a permission error.
You can fix this by creating the following file:

$ cat /etc/udev/rules.d/50-esp.rules
# ESP32-C3
SUBSYSTEMS=="usb", ATTRS{idVendor}=="303a", ATTRS{idProduct}=="1001", MODE="0666"

For more details, see:
https://docs.espressif.com/projects/esp-idf/en/latest/esp32c3/api-guides/jtag-debugging/index.html

[aykevl]

This PR also includes "serial-port": ["acm:303a:1001"] which avoids errors with multiple serial ports and makes sure to flash over the built-in USB-to-serial converter at port 18 and 19 of the ESP32-C3 (both would work, but adding this serial port makes sure only one is picked).

[deadprogram]

🐛 thanks @aykevl now merging.