esp32c3: add support for GDB debugging

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

targets/esp32c3.json

@@ -11,6 +11,10 @@
 		"src/device/esp/esp32c3.S"
 	],
 	"binary-format": "esp32c3",
-	"flash-command": "esptool.py --chip=esp32c3 --port {port} write_flash 0x0 {bin}"
+	"flash-command": "esptool.py --chip=esp32c3 --port {port} write_flash 0x0 {bin}",
+	"serial-port": ["acm:303a:1001"],
+	"openocd-interface": "esp_usb_jtag",
+	"openocd-target": "esp32c3",
+	"openocd-commands": ["gdb_memory_map disable"]
 }
 

targets/riscv32.json

@@ -8,5 +8,6 @@
 	],
 	"ldflags": [
 		"-melf32lriscv"
-	]
+	],
+	"gdb": ["gdb-multiarch"]
 }