[libc] Add abs()

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/stdlib.h

  ****************************************************************************
  */
 
+static inline __attribute__ (( always_inline )) int abs ( int value ) {
+	return __builtin_abs ( value );
+}
+
 extern int system ( const char *command );
 extern __asmcall int main ( void );