Don't use prebuilt rust stdlibs for linux bionic

Linux bionic doesn't have rust stdlib prebuilts, don't use the prebuilts
when it is configured as the host os.

Bug: 216192129
Test: builds
Change-Id: I03425dee4b2e3659bf90c0a0c808cc286daa4b7f
diff --git a/rust/compiler.go b/rust/compiler.go
index 365f1e0..bf6a488 100644
--- a/rust/compiler.go
+++ b/rust/compiler.go
@@ -371,8 +371,9 @@
 
 	if !Bool(compiler.Properties.No_stdlibs) {
 		for _, stdlib := range config.Stdlibs {
-			// If we're building for the build host, use the prebuilt stdlibs
-			if ctx.Host() && !ctx.Target().HostCross {
+			// If we're building for the build host, use the prebuilt stdlibs, unless the host
+			// is linux_bionic which doesn't have prebuilts.
+			if ctx.Host() && !ctx.Target().HostCross && ctx.Target().Os != android.LinuxBionic {
 				stdlib = "prebuilt_" + stdlib
 			}
 			deps.Stdlibs = append(deps.Stdlibs, stdlib)