hifive1b: add support for gdb subcommand

This makes debugging on the HiFive1 rev B much easier:

    tinygo gdb -target=hifive1b examples/echo

Using JLinkGDBServer as I couldn't figure out how to do it with OpenOCD.

targets/hifive1b.json

@@ -4,5 +4,6 @@
 	"linkerscript": "targets/hifive1b.ld",
 	"flash-method": "msd",
 	"msd-volume-name": "HiFive",
-	"msd-firmware-name": "firmware.hex"
+	"msd-firmware-name": "firmware.hex",
+	"jlink-device": "fe310"
 }