Change v8_snapshot_toolchain to be a GN build arg.

It is likely that CrOS and other distros will want to be
able to build the v8 snapshot with a custom toolchain.

This CL makes snapshot_toolchain a declared build arg, and
renames it to v8_snapshot_toolchain to minimize the risk
of a name collision.

R=machenbach@chromium.org, jochen@chromium.org
BUG=608596

Review-Url: https://codereview.chromium.org/1969693002
Cr-Commit-Position: refs/heads/master@{#36193}

snapshot_toolchain.gni

@@ -25,22 +25,40 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-# The snapshot needs to be compiled for the host, but compiled with
-# a toolchain that matches the bit-width of the target.
+declare_args() {
+  # The snapshot needs to be compiled for the host, but compiled with
+  # a toolchain that matches the bit-width of the target.
+  v8_snapshot_toolchain = ""
+}
 
 # 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" && host_os == "linux") {
-  if (target_cpu == "arm" || target_cpu == "mipsel" || target_cpu == "x86") {
-    snapshot_toolchain = "//build/toolchain/linux:clang_x86"
-  } else if (target_cpu == "x64" || target_cpu == "arm64" || target_cpu == "mips64el") {
-    snapshot_toolchain = "//build/toolchain/linux:clang_x64"
+if (v8_snapshot_toolchain == "") {
+  if (host_cpu == "x64" && host_os == "linux") {
+    if (target_cpu == "arm" || target_cpu == "mipsel" || target_cpu == "x86") {
+      if (target_os == "android" || is_clang) {
+        v8_snapshot_toolchain = "//build/toolchain/linux:clang_x86"
+      } else {
+        v8_snapshot_toolchain = "//build/toolchain/linux:x86"
+      }
+
+    } else if (target_cpu == "x64" || target_cpu == "arm64" ||
+               target_cpu == "mips64el") {
+      if (target_os == "android" || is_clang) {
+        v8_snapshot_toolchain = "//build/toolchain/linux:clang_x64"
+      } else {
+        v8_snapshot_toolchain = "//build/toolchain/linux:x64"
+      }
+    } else {
+      assert(false, "Need environment for this arch: $target_cpu")
+    }
   } else {
-    assert(false, "Need environment for this arch: $target_cpu")
+    v8_snapshot_toolchain = default_toolchain
   }
-} else {
-  snapshot_toolchain = default_toolchain
 }
 
-
+# TODO(GYP): snapshot_toolchain is provided for backwards compatibility
+# and should be removed once all callers are updated to refer to
+# v8_snapshot_toolchain directly.
+snapshot_toolchain = v8_snapshot_toolchain