Do not add non-existing -Bprebuilts/gcc/.../bin

* Only prebuilts/gcc/linux-x86/host/x86_64-w64-mingw32-4.8/x86_64-w64-mingw32/bin
  exists for x86_windows_host.
* Remove config.ToolPath;
  add required -B flags into ToolchainCflags and ToolchainLdflags.

Bug: 218883919
Test: make droid tidy-soong_subset
Change-Id: I9a18bf8cc0cf84e091c7463b3bda316eaab53aa3
diff --git a/cc/config/darwin_host.go b/cc/config/darwin_host.go
index 206bec1..5e3f7c7 100644
--- a/cc/config/darwin_host.go
+++ b/cc/config/darwin_host.go
@@ -258,8 +258,12 @@
 	return darwinAvailableLibraries
 }
 
-func (t *toolchainDarwin) ToolPath() string {
-	return "${config.MacToolPath}"
+func (t *toolchainDarwin) ToolchainCflags() string {
+	return "-B${config.MacToolPath}"
+}
+
+func (t *toolchainDarwin) ToolchainLdflags() string {
+	return "-B${config.MacToolPath}"
 }
 
 var toolchainDarwinArmSingleton Toolchain = &toolchainDarwinArm{}