rust: set STD_ENV_ARCH

In 1.47.0, std::env requires the STD_ENV_ARCH environment variable to be
set to the target architecture [1].

[1] 8e8939b804/library/std/src/env.rs (L866)

Bug: 171280890
Test: cd external/rust; mm
Change-Id: If4ddf4b9bad2a1a7518a358a5fb597170a97c67e

rust/builder.go

@@ -22,6 +22,7 @@ import (
 
 	"android/soong/android"
 	"android/soong/cc"
+	"android/soong/rust/config"
 )
 
 var (
@@ -138,6 +139,13 @@ func transformSrctoCrate(ctx ModuleContext, main android.Path, deps PathDeps, fl
 	crate_name := ctx.RustModule().CrateName()
 	targetTriple := ctx.toolchain().RustTriple()
 
+	// libstd requires a specific environment variable to be set. This is
+	// not officially documented and may be removed in the future. See
+	// https://github.com/rust-lang/rust/blob/master/library/std/src/env.rs#L866.
+	if crate_name == "std" {
+		envVars = append(envVars, "STD_ENV_ARCH="+config.StdEnvArch[ctx.RustModule().Arch().ArchType])
+	}
+
 	inputs = append(inputs, main)
 
 	// Collect rustc flags