Pass the `--cfg soong` flag when building rustdoc

This CL modifies Soong to pass the `--cfg soong` flag to the rustdoc
tool.

Test: m rustdoc
Bug: 303116432
Change-Id: I089edebe3c0450b9f9d4f97404bafb7e90ed1f8a
diff --git a/rust/builder.go b/rust/builder.go
index 1ce92f4..f469f56 100644
--- a/rust/builder.go
+++ b/rust/builder.go
@@ -520,6 +520,9 @@
 	// this flag.
 	rustdocFlags = append(rustdocFlags, "-Z", "unstable-options", "--enable-index-page")
 
+	// Ensure we use any special-case code-paths for Soong.
+	rustdocFlags = append(rustdocFlags, "--cfg", "soong")
+
 	targetTriple := ctx.toolchain().RustTriple()
 
 	// Collect rustc flags