Make sure to pass any flags for SIMD to test_simd.cpp if using the ninja
generator.

https://bugs.gentoo.org/969037
https://bugs.gentoo.org/969038
--- a/src/build-data/ninja.in
+++ b/src/build-data/ninja.in
@@ -221,6 +221,7 @@ build %{obj}: compile_exe %{src}
 
 %{for test_build_info}
 build %{obj}: compile_exe %{src}
+  isa_flags = %{isa_flags}
 %{endfor}
 
 %{for fuzzer_build_info}