[liba] Add abs function for int

Change-Id: Ic133ff2f34c7ff24ecbec69d5f31e447a72ae4fe

liba/Makefile

@@ -3,6 +3,7 @@ SFLAGS += -Iliba/include
 liba_src += $(addprefix liba/src/, \
   armv7m/setjmp.s \
   armv7m/longjmp.s \
+  abs.c \
   assert.c \
   bzero.c \
   ctype.c \

liba/include/stdlib.h

@@ -13,6 +13,8 @@ void * calloc(size_t count, size_t size);
 
 void abort(void) __attribute__((noreturn));
 
+int abs(int n);
+
 LIBA_END_DECLS
 
 #endif

liba/src/abs.c

@@ -0,0 +1,5 @@
+#include <stdlib.h>
+
+int abs(int n) {
+  return n < 0 ? -n : n;
+}