Add gpio.{h, c}

Makefile

@@ -8,6 +8,8 @@ CFLAGS += -ffunction-sections -fdata-sections
 CFLAGS += -fno-builtin
 # "-fno-common ... specifies that the compiler places uninitialized global variables in the BSS section of the object file."
 CFLAGS += -fno-common
+# Use experimental binary literal gcc extension (to be able to use binary literals; e.g. 0b0101)
+CFLAGS += -std=c2x
 
 DEBUG ?= 1
 ifeq ($(DEBUG), 0)