rust: Use prebuilts for x86 host target.

Use prebuilts for x86 host targets to make both x86 and x86_64 builds
more consistent with one another. This fixes building x86 binaries.

Bug: 162063992
Bug: 169347277
Test: Build 32-bit host module.
Change-Id: Id11eb0f3b949439cec090060a33b755af94b5883
diff --git a/rust/compiler.go b/rust/compiler.go
index 586063e..224e201 100644
--- a/rust/compiler.go
+++ b/rust/compiler.go
@@ -254,7 +254,7 @@
 	if !Bool(compiler.Properties.No_stdlibs) {
 		for _, stdlib := range config.Stdlibs {
 			// If we're building for the primary arch of the build host, use the compiler's stdlibs
-			if ctx.Target().Os == android.BuildOs && ctx.TargetPrimary() {
+			if ctx.Target().Os == android.BuildOs {
 				stdlib = stdlib + "_" + ctx.toolchain().RustTriple()
 			}