help: list -showcc, -w (#7013)

cmd/v/help/build-c.txt

@@ -59,5 +59,8 @@ These build flags are enabled on `build` and `run` as long as the backend is set
       Useful for similar use-case as above option, except it replaces V-generated prelude with
       your custom one loaded from specified <path>.
 
+   -showcc
+      Prints the C command that is used to build the program.
+
    -keepc
       Do not remove the temporary .tmp.c and .tmp.c.rsp files. Also do not use a random prefix for them, so they would be fixed and predictable.

cmd/v/help/build.txt

@@ -119,6 +119,9 @@ The build flags are shared by the build and run commands:
          C GEN: 103ms
          C tcc: 95ms
 
+   -w
+      Hide all warnings.
+
    -W
       Treat all warnings as errors, even in development builds.