Add support for SPIRV writing to BUILD.gn

BUG=tint:48

Change-Id: I6b92bb372b783ad4498fdc4817d0893851b0bb90
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/19680
Reviewed-by: dan sinclair <dsinclair@google.com>
diff --git a/tint_overrides_with_defaults.gni b/tint_overrides_with_defaults.gni
index dc7d731..7fb33e5 100644
--- a/tint_overrides_with_defaults.gni
+++ b/tint_overrides_with_defaults.gni
@@ -41,6 +41,11 @@
     tint_build_spv_reader = false
   }
 
+  # Build the SPIR-V output writer
+  if (!defined(tint_build_spv_writer)) {
+    tint_build_spv_writer = false
+  }
+
   # TODO(rharrison): Implement support for the reset of the reader/writers
 
   # Generate fuzzers