cpp: don't handroll an incorrect strtoul(3c), use the one in libc
The previous implementation would parse 0x7ff as 0x755 (etc).

*** 51,60 ****
--- 51,62 ----
          $(LINK.prog)
  
  %.o$(STRAP): %.c
          $(COMPILE.c)
  
+ y.tab.o$(STRAP): yylex.c
+ 
  #
  # We need to distinguish between the cpp build in the bootstrap and the cpp
  # built normally. However, when we install it, they need to have the same name.
  # To handle this we add a small bit of shell logic. Note that the mv bit is
  # explicitly ignored and instead we do a final check to make sure we have