[libc] Add abs() function

libc/include/stdlib.h

@@ -33,6 +33,7 @@ void dtostr(double val, int style, int preci, char *buf);
 
 long atol(const char *str);
 int atoi(const char *str);
+int abs(int x);
 
 /* Returned by `div'.  */
 typedef struct

libc/misc/Makefile

@@ -6,6 +6,7 @@ LIB ?= out.a
 include $(TOPDIR)/libc/$(COMPILER).inc
 
 OBJS = \
+	abs.o \
 	aliases.o \
 	atexit.o \
 	atof.o \

libc/misc/abs.c

@@ -0,0 +1,6 @@
+#include <stdlib.h>
+
+int abs(int x)
+{
+    return x < 0? -x : x;
+}