mksnapshot: introduce a V8_TARGET_OS_CHROMEOS define

We had one use of OS_CHROMEOS in mksnapshot. OS_CHROMEOS is defined if
gn's `is_chromeos` is true, which checks `current_os`. `current_os !=
target_os` can happen if we're building with a non-default toolchain,
which happens often on CrOS, since `mksnapshot` is a host binary.

Tested by manually verifying that .text.hot.embedded now shows up on
arm32/aarch64 builds of embedded.S.

Bug: v8:9103
Change-Id: I038b56f4c18c7dd9a651ce676a977697dad14ae6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1573041
Commit-Queue: George Burgess <gbiv@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60925}

BUILD.gn

@@ -2977,6 +2977,7 @@ v8_source_set("v8_base_without_compiler") {
       "src/x64/register-x64.h",
       "src/x64/sse-instr.h",
     ]
+
     # iOS Xcode simulator builds run on an x64 target. iOS and macOS are both
     # based on Darwin and thus POSIX-compliant to a similar degree.
     if (is_linux || is_mac || is_ios) {
@@ -3738,6 +3739,8 @@ if (v8_use_snapshot && current_toolchain == v8_snapshot_toolchain) {
       defines = [ "V8_TARGET_OS_FUCHSIA" ]
     } else if (target_os == "win") {
       defines = [ "V8_TARGET_OS_WIN" ]
+    } else if (target_os == "chromeos") {
+      defines = [ "V8_TARGET_OS_CHROMEOS" ]
     }
   }
 }

src/snapshot/embedded-file-writer.cc

@@ -680,7 +680,7 @@ int PlatformDependentEmbeddedFileWriter::IndentedDataDirective(
 #else
 
 void PlatformDependentEmbeddedFileWriter::SectionText() {
-#ifdef OS_CHROMEOS
+#if defined(V8_TARGET_OS_CHROMEOS)
   fprintf(fp_, ".section .text.hot.embedded\n");
 #else
   fprintf(fp_, ".section .text\n");