esp: add picolibc

With this change, it is possible to fully use CGo on ESP32/ESP8266
chips. The following will work:

    tinygo flash -target=d1mini       -port=/dev/ttyUSB0 ./testdata/cgo/
    tinygo flash -target=esp32-mini32 -port=/dev/ttyUSB0 ./testdata/cgo/

Previously it would produce output like the following:

    /tmp/tinygo905539688/main.o:(.literal.runtime.run$1$gowrapper+0x150): undefined reference to `strcpy'
    /tmp/tinygo905539688/main.o:(.literal.runtime.run$1$gowrapper+0x154): undefined reference to `strlen'

targets/esp32.json

@@ -9,6 +9,7 @@
 		"-mcpu=esp32"
 	],
 	"rtlib": "compiler-rt",
+	"libc": "picolibc",
 	"linkerscript": "targets/esp32.ld",
 	"extra-files": [
 		"src/device/esp/esp32.S",

targets/esp8266.json

@@ -9,6 +9,7 @@
 		"-mcpu=esp8266"
 	],
 	"rtlib": "compiler-rt",
+	"libc": "picolibc",
 	"linkerscript": "targets/esp8266.ld",
 	"extra-files": [
 		"src/device/esp/esp8266.S",