Add x86_host vars to config.bzl

Test: USE_BAZEL_ANALYSIS=1 m adbd
Test: Manually verified config.bzl contains various x86_host flags after
bp2build.
Test: Unit tests

Change-Id: Ie9201ea2be4cd1c6659bea088a797cedbae37403
diff --git a/cc/config/global.go b/cc/config/global.go
index 6108d3d..7abd2fc 100644
--- a/cc/config/global.go
+++ b/cc/config/global.go
@@ -404,7 +404,7 @@
 	pctx.StaticVariableWithEnvOverride("REAbiLinkerExecStrategy", "RBE_ABI_LINKER_EXEC_STRATEGY", remoteexec.LocalExecStrategy)
 }
 
-var HostPrebuiltTag = pctx.VariableConfigMethod("HostPrebuiltTag", android.Config.PrebuiltOS)
+var HostPrebuiltTag = exportVariableConfigMethod("HostPrebuiltTag", android.Config.PrebuiltOS)
 
 func ClangPath(ctx android.PathContext, file string) android.SourcePath {
 	type clangToolKey string