cc: add comment about -S
 [rsc] --rw-rw-r-- M 1276246 glenda sys 24200 Mar 27 07:41 sys/src/cmd/cc/lex.c
	/n/sourcesdump/2006/0327/plan9/sys/src/cmd/cc/lex.c:24,29 - 
	/n/sourcesdump/2006/0328/plan9/sys/src/cmd/cc/lex.c:24,30
	   *	-p		use standard cpp ANSI preprocessor (not on windows)
	   *	-r		print registerization
	   *	-s		print structure offsets (with -a or -aa)
	+  *	-S		print assembly
	   *	-t		print type trees
	   *	-V		enable void* conversion warnings
	   *	-v		verbose printing