gba: add support for mGBA debugging

Implements `putchar` when `mgbadebug` build tag is set which outputs
text through the mGBA debugging output interface.

mGBA allows the games running in the emulator to output debug text the
process to do so is:
1. set 0x4FFF780 to value of 0xC0DE
2. write text in memory 0x4FFF600 to 0x4FFF700
3. set 0x4FFF700 to the desired log level value from 0x100 (fatal) to
0x104 (debug)

Once this is done mGBA will output the text in its logs view as well as
through stdout. (Setting the log level to fatal also produces a dialog
box with the text)

The text output in the CLI is prefixed with `[DEBUG] GBA Debug: ` so I
modified the regex used for address matching in panic messages to be
able to read the address when the output line doesn't start with
`panic`.

monitor.go

@@ -278,7 +278,7 @@ func ListSerialPorts() ([]SerialPortInfo, error) {
 	return serialPortInfo, nil
 }
 
-var addressMatch = regexp.MustCompile(`^panic: runtime error at 0x([0-9a-f]+): `)
+var addressMatch = regexp.MustCompile(`panic: runtime error at 0x([0-9a-f]+): `)
 
 // Extract the address from the "panic: runtime error at" message.
 func extractPanicAddress(line []byte) uint64 {