Don't assert that we only cross compile on linux

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 827cd2c366bbdf2a7c4d3cf8792a78b102b7ee2c..8a01e8663b8a8befca740401b3bd6e513596d2f4 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -32,9 +32,8 @@ v8_toolset_for_d8 = "host"
 # TODO(GYP): For now we only support 32-bit little-endian target builds from an
 # x64 Linux host. Eventually we need to support all of the host/target
 # configurations v8 runs on.
-if (host_cpu == "x64" &&
+if (host_cpu == "x64" && host_os == "linux" &&
     (target_cpu == "arm" || target_cpu == "mipsel" || target_cpu == "x86")) {
-  assert(host_os == "linux")
   snapshot_toolchain = "//build/toolchain/linux:clang_x86"
 } else {
   snapshot_toolchain = default_toolchain