esp8266: add compiler-rt library

With this change, it is possible to compile ./testdata/float.go for the
ESP8266 and run it successfully. Previously it would result in many
linker error like this:

    /tmp/tinygo494494333/main.o:(.literal.runtime.printfloat64+0x0): undefined reference to `__unorddf2'
    /tmp/tinygo494494333/main.o:(.literal.runtime.printfloat64+0x4): undefined reference to `__gtdf2'
    /tmp/tinygo494494333/main.o:(.literal.runtime.printfloat64+0xc): undefined reference to `__nedf2'
    /tmp/tinygo494494333/main.o:(.literal.runtime.printfloat64+0x10): undefined reference to `__ltdf2'
    /tmp/tinygo494494333/main.o:(.literal.runtime.printfloat64+0x1c): undefined reference to `__gedf2'

I have verified that the output on the serial console matches
./testdata/float.txt when run on the ESP8266.

targets/esp8266.json

@@ -8,6 +8,7 @@
 	"cflags": [
 		"-mcpu=esp8266"
 	],
+	"rtlib": "compiler-rt",
 	"linkerscript": "targets/esp8266.ld",
 	"extra-files": [
 		"src/device/esp/esp8266.S",