Land chromium-side work to clean up handling of v8_target_cpu in the GN build.

build/config/sanitizers/sanitizers.gni

Index: build/config/sanitizers/sanitizers.gni
diff --git a/build/config/sanitizers/sanitizers.gni b/build/config/sanitizers/sanitizers.gni
index 32cecffcae1e3ecf9ba178625a873add8a96833a..dddd9938381b635f1661732bba57d379557dd0e1 100644
--- a/build/config/sanitizers/sanitizers.gni
+++ b/build/config/sanitizers/sanitizers.gni
@@ -9,8 +9,8 @@ declare_args() {
   # Compile for Leak Sanitizer to find leaks.
   is_lsan = false
 
-  # Compile for Memory Sanitizer to find uninitialized reads.
-  is_msan = false
+  # is_msan is declared in //build/config/BUILDCONFIG.gn; see the comments
+  # in that file for why that is.
 
   # Compile for Thread Sanitizer to find threading bugs.
   is_tsan = false
@@ -128,3 +128,6 @@ prebuilt_instrumented_libraries_available =
 # For one-off testing, just comment this assertion out.
 assert(!is_debug || !(is_msan || is_ubsan || is_ubsan_null || is_ubsan_vptr),
        "Sanitizers should generally be used in release (set is_debug=false).")
+
+assert(!is_msan || (is_linux && current_cpu == "x64"),
+       "MSan currently only works on 64-bit Linux and ChromeOS builds.")