v help: add a `v help build-native` section (#9906)

cmd/v/help/build-native.txt

@@ -0,0 +1,17 @@
+Usage: v -native [-options] ['run'] <target.v|target_directory> [run options]
+
+This command compiles the given target, along with their dependencies, into an executable using the Native code generation backend.
+
+For more general build help, see also `v help build`.
+
+# Interfacing the Native code generation, passing options to it:
+   -v
+      Display the assembly code generated (that may change to `-showasm` in the future)
+
+   -arch <arch>
+      Select target architecture, right now only `arm64` and `amd64` are supported
+
+   -os <os>, -target-os <os>
+      Change the target OS that V compiles for.
+
+      The supported targets for the native backend are: `macos`, `linux`

cmd/v/help/build.txt

@@ -155,5 +155,7 @@ NB: the build flags are shared with the run command too:
 
 For C-specific build flags, use `v help build-c`.
 
+For Native-specific build flags, use `v help build-native`.
+
 See also:
    `v help run` for documentation regarding `v run`.