pref: add -show-timings option

cmd/v/help/build.txt

@@ -110,6 +110,16 @@ The build flags are shared by the build and run commands:
       to detect whether or not to use ANSI colors may not work in all cases.
       These options allow you to override the default detection.
 
+   -check-syntax
+      Only scan and parse the files, but then stop. Useful for very quick syntax checks.
+   
+   -show-timings
+      Print a summary about how long each compiler stage took, for example:
+         PARSE: 152ms
+         CHECK: 62ms
+         C GEN: 103ms
+         C tcc: 95ms
+      
 For C-specific build flags, use `v help build-c`.
 
 See also: