Add support for preferred arch symlinks

Add a symlink_preferred_arch property to binaries to allow compiling the
binary for multiple architectures and then creating a symlink to the
preferred archicture, for example dalvikvm32 and dalvikvm64, with
dalvikvm symlinked to dalvikvm64.

Test: mmma -j art/dalvikvm
Change-Id: Ied15f2be9d52c01006fe8ac207c175b78558eab1
diff --git a/android/variable.go b/android/variable.go
index 531125d..be2407d 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -102,6 +102,9 @@
 	Schedboost                 *bool `json:",omitempty"`
 	Binder32bit                *bool `json:",omitempty"`
 
+	DevicePrefer32BitExecutables *bool `json:",omitempty"`
+	HostPrefer32BitExecutables   *bool `json:",omitempty"`
+
 	SanitizeHost   *[]string `json:",omitempty"`
 	SanitizeDevice *[]string `json:",omitempty"`
 }