Revert of Use GCC for snapshot_toolchain when is_clang=false.

snapshot_toolchain.gni

Index: snapshot_toolchain.gni
diff --git a/snapshot_toolchain.gni b/snapshot_toolchain.gni
index 4d305a9b2dcd19b8aa29bedc2bd51596556cd0c0..4932110489dd1a7fe65a6124aa655192661d3bd6 100644
--- a/snapshot_toolchain.gni
+++ b/snapshot_toolchain.gni
@@ -33,17 +33,9 @@
 # configurations v8 runs on.
 if (host_cpu == "x64" && host_os == "linux") {
   if (target_cpu == "arm" || target_cpu == "mipsel" || target_cpu == "x86") {
-    if (is_clang) {
-      snapshot_toolchain = "//build/toolchain/linux:clang_x86"
-    } else {
-      snapshot_toolchain = "//build/toolchain/linux:x86"
-    }
+    snapshot_toolchain = "//build/toolchain/linux:clang_x86"
   } else if (target_cpu == "x64" || target_cpu == "arm64" || target_cpu == "mips64el") {
-    if (is_clang) {
-      snapshot_toolchain = "//build/toolchain/linux:clang_x64"
-    } else {
-      snapshot_toolchain = "//build/toolchain/linux:x64"
-    }
+    snapshot_toolchain = "//build/toolchain/linux:clang_x64"
   } else {
     assert(false, "Need environment for this arch: $target_cpu")
   }