Lunch: don't use a toolchain that doesn't exist.

"lunch sdk-eng" on cygwin tries to use a gcc.exe from the
prebuild arm toolchain, but it doesn't exist. This prevents
from setting variables to point on a gcc toolchain if there
is none for the given arch.

Change-Id: I3b72220663687f9fdaacc050899aef00632f2c3c
diff --git a/core/dumpvar.mk b/core/dumpvar.mk
index 0c58559..58cdc6d 100644
--- a/core/dumpvar.mk
+++ b/core/dumpvar.mk
@@ -8,8 +8,11 @@
 ifeq ($(TARGET_SIMULATOR),true)
 	ABP:=$(ABP):$(TARGET_OUT_EXECUTABLES)
 else
-	# this should be copied to HOST_OUT_EXECUTABLES instead
-	ABP:=$(ABP):$(PWD)/prebuilt/$(HOST_PREBUILT_TAG)/toolchain/arm-eabi-4.4.3/bin
+	# Add the toolchain bin dir if it actually exists
+	ifneq ($(wildcard $(PWD)/prebuilt/$(HOST_PREBUILT_TAG)/toolchain/arm-eabi-4.4.3/bin),)
+		# this should be copied to HOST_OUT_EXECUTABLES instead
+		ABP:=$(ABP):$(PWD)/prebuilt/$(HOST_PREBUILT_TAG)/toolchain/arm-eabi-4.4.3/bin
+	endif
 endif
 ANDROID_BUILD_PATHS := $(ABP)
 ANDROID_PREBUILTS := prebuilt/$(HOST_PREBUILT_TAG)