cgen: support -profile - and -profile x.txt

cmd/v/internal/help/build.txt

@@ -65,8 +65,15 @@ The build flags are shared by the build and run commands:
    -prod
       Compile the executable in production mode where most optimizations are enabled.
 
-   -prof, -profile
+   -prof, -profile <file.txt>
       Compile the executable with all functions profiled.
+      The profile results will be stored in `file.txt`.
+      The format is 4 fields, separated by a space, for each v function:
+         a) how many times it was called
+         b) how much *nanoseconds in total* it took
+         c) an average for each function (i.e. (b) / (a) )
+         d) the function name
+      NB: if you want to output the profile info to stdout, use `-profile -`.
 
    -stats
       Enable more detailed statistics reporting, while compiling test files.

cmd/v/v.v

@@ -146,7 +146,9 @@ fn parse_args(args []string) (&pref.Preferences, string) {
 				res.is_bare = true
 			}
 			'-prof', '-profile' {
+				res.profile_file = cmdline.option(current_args, '-profile', '-')
 				res.is_prof = true
+				i++
 			}
 			'-prod' {
 				res.is_prod = true