Use GCC for snapshot_toolchain when is_clang=false.

Currently, snapshot_toolchain is hardcoded to use a clang host
toolchain. Use a GCC toolchain if is_clang is false.

Revert this when this is root-caused (see crbug.com/601486)

LOG=Y
BUG=601486

Review URL: https://codereview.chromium.org/1809643003

Cr-Commit-Position: refs/heads/master@{#35341}

snapshot_toolchain.gni

@@ -33,9 +33,17 @@
 # configurations v8 runs on.
 if (host_cpu == "x64" && host_os == "linux") {
   if (target_cpu == "arm" || target_cpu == "mipsel" || target_cpu == "x86") {
-    snapshot_toolchain = "//build/toolchain/linux:clang_x86"
+    if (is_clang) {
+      snapshot_toolchain = "//build/toolchain/linux:clang_x86"
+    } else {
+      snapshot_toolchain = "//build/toolchain/linux:x86"
+    }
   } else if (target_cpu == "x64" || target_cpu == "arm64" || target_cpu == "mips64el") {
-    snapshot_toolchain = "//build/toolchain/linux:clang_x64"
+    if (is_clang) {
+      snapshot_toolchain = "//build/toolchain/linux:clang_x64"
+    } else {
+      snapshot_toolchain = "//build/toolchain/linux:x64"
+    }
   } else {
     assert(false, "Need environment for this arch: $target_cpu")
   }