Check USE_GOMA consistently

Currently, the only way to properly disable goma is to unset USE_GOMA.
If you set USE_GOMA=false, we won't turn on goma, but we'll pass -j500
to ninja.

Test: USE_GOMA={x,false} m -j48 showcommands (inspect)
Change-Id: I6b327d08ebadbe614a3bf7646fd597054bbfaaa2
diff --git a/core/ninja.mk b/core/ninja.mk
index 27441ad..533553e 100644
--- a/core/ninja.mk
+++ b/core/ninja.mk
@@ -114,7 +114,7 @@
 # proceeding with undefined behavior.
 NINJA_EXTRA_ARGS += -w dupbuild=err
 
-ifdef USE_GOMA
+ifneq ($(filter-out false,$(USE_GOMA)),)
 KATI_MAKEPARALLEL := $(MAKEPARALLEL)
 # Ninja runs remote jobs (i.e., commands which contain gomacc) with
 # this parallelism. Note the parallelism of all other jobs is still