libc: adjust brk(0) to return the existing break, and use it to initialize sbrk()

@@ -35,7 +35,7 @@
  */
 
         ENTRY_NP(_brk_unlocked)
         SYSTRAP_RVAL1(brk)
         SYSCERROR
-        RETC
+        RET
         SET_SIZE(_brk_unlocked)