set a prebuilt directory name for the 2nd arch

In order to install 32-bit gdbserver on a 64-bit build,
prebuilts/misc/Android.mk needs to know the prebuilt directory name
for the 2nd arch.

Change-Id: Ibc943b758eb9404f7f5f9fbbd8720cc67f6a6f01
diff --git a/core/config.mk b/core/config.mk
index b91ff7f..7150380 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -579,6 +579,9 @@
 # This is the standard way to name a directory containing prebuilt target
 # objects. E.g., prebuilt/$(TARGET_PREBUILT_TAG)/libc.so
 TARGET_PREBUILT_TAG := android-$(TARGET_ARCH)
+ifdef TARGET_2ND_ARCH
+TARGET_2ND_PREBUILT_TAG := android-$(TARGET_2ND_ARCH)
+endif
 
 # Set up RS prebuilt variables for compatibility library