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

@@ -51,10 +51,12 @@
         $(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