Don't assert that we only cross compile on linux

We also cross compile on Windows for example. Since we can't switch the
toolchain there, we'll just use the 64bit toolchain and produce an
invalid snapshot :-/

BUG=none
R=dpranke@chromium.org
LOG=n
TBR=machenbach@chromium.org

Review URL: https://codereview.chromium.org/1008933002

Cr-Commit-Position: refs/heads/master@{#27192}

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