Add support for different host/target platform in mksnapshot.

This Mac/Fuchsia build was broken as the generated assembly file must
follow Linux-style calling conventions.

Change-Id: I05d946e77690e97f05d57a3d5ba04cd2e63fbb9d
Reviewed-on: https://chromium-review.googlesource.com/c/1377230
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58507}

BUILD.gn

@@ -3501,6 +3501,10 @@ if (v8_use_snapshot && current_toolchain == v8_snapshot_toolchain) {
       ":v8_nosnapshot",
       "//build/win:default_exe_manifest",
     ]
+
+    if (target_os == "fuchsia") {
+      defines = [ "V8_TARGET_OS_FUCHSIA" ]
+    }
   }
 }
 

src/snapshot/embedded-file-writer.cc

@@ -81,9 +81,11 @@ const char* DirectiveAsString(DataDirective directive) {
 }
 
 // V8_OS_MACOSX
+// Fuchsia target is explicitly excluded here for Mac hosts. This is to avoid
+// generating uncompilable assembly files for the Fuchsia target.
 // -----------------------------------------------------------------------------
 
-#if defined(V8_OS_MACOSX)
+#if defined(V8_OS_MACOSX) && !defined(V8_TARGET_OS_FUCHSIA)
 
 void PlatformDependentEmbeddedFileWriter::SectionText() {
   fprintf(fp_, ".text\n");