Enable goma in soong

Pass the USE_GOMA flag to soong, and export the CC_WRAPPER environment
variable after adding gomacc to it.

Bug: 31142427
Change-Id: I6de698a40817c0615b03fa17bd27075465d79ffd
diff --git a/core/soong.mk b/core/soong.mk
index 80dcefd..29a91e2 100644
--- a/core/soong.mk
+++ b/core/soong.mk
@@ -54,6 +54,7 @@
 	echo '    "Binder32bit": $(if $(BINDER32BIT),true,false),'; \
 	echo '    "DevicePrefer32BitExecutables": $(if $(filter true,$(TARGET_PREFER_32_BIT_EXECUTABLES)),true,false),'; \
 	echo '    "HostPrefer32BitExecutables": $(if $(filter true,$(HOST_PREFER_32_BIT)),true,false),'; \
+	echo '    "UseGoma": $(if $(filter-out false,$(USE_GOMA)),true,false),'; \
 	echo ''; \
 	echo '    "DeviceName": "$(TARGET_DEVICE)",'; \
 	echo '    "DeviceArch": "$(TARGET_ARCH)",'; \