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

cc/cc_test.go

@@ -3943,7 +3943,6 @@ func TestIncludeDirectoryOrdering(t *testing.T) {
 		"${config.ArmGenericCflags}",
 		"-target",
 		"armv7a-linux-androideabi20",
-		"-B${config.ArmGccRoot}/arm-linux-androideabi/bin",
 	}
 
 	expectedIncludes := []string{