compiler: implement -W and -reuse-tmpc

cmd/v/help/build-c.txt

@@ -58,3 +58,6 @@ These build flags are enabled on `build` and `run` as long as the backend is set
    -custom-prelude <path>
       Useful for similar use-case as above option, except it replaces V-generated prelude with
       your custom one loaded from specified <path>.
+
+   -reuse-tmpc
+      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

@@ -118,6 +118,9 @@ The build flags are shared by the build and run commands:
          C GEN: 103ms
          C tcc: 95ms
 
+   -W
+      Treat all warnings as errors, even in development builds.
+
 For C-specific build flags, use `v help build-c`.
 
 See also: