Update rustc manpage.

rustc -C target-cpu=help is no longer supported. Recommend the
llc tool intead like 'rustc --help'.

man/rustc.1

@@ -160,7 +160,7 @@ If the value is 'help', then a list of available CPUs is printed.
 \fBtarget\-feature\fR='\fI+feature1\fR,\fI\-feature2\fR'
 A comma\[hy]separated list of features to enable or disable for the target.
 A preceding '+' enables a feature while a preceding '\-' disables it.
-Available features can be discovered through \fItarget\-cpu=help\fR.
+Available features can be discovered through \fIllc -mcpu=help\fR.
 .TP
 \fBpasses\fR=\fIval\fR
 A space\[hy]separated list of extra LLVM passes to run.