build: add support for armasm64.exe

armasm64.exe is Microsoft's reference compiler for 64-bit Arm assembly code.
This is needed to make sure that V8 can be tested and cross-compiled
under MSVC for Windows on Arm systems.

Bug: v8:10012
Change-Id: Id1b9ab7bb0cc96179a707e9746f9e78e8da30de9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2071941
Commit-Queue: Richard Townsend <richard.townsend@arm.com>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Auto-Submit: Richard Townsend <richard.townsend@arm.com>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747010}

build/toolchain/win/BUILD.gn

@@ -221,21 +221,40 @@ template("msvc_toolchain") {
     }
 
     tool("asm") {
+      is_msvc_assembler = true
+
       if (toolchain_args.current_cpu == "arm64") {
-        prefix = rebase_path("$clang_base_path/bin", root_build_dir)
-        ml = "${clang_prefix}${prefix}/${clang_cl} --target=arm64-windows"
-        if (host_os == "win") {
-          # Flip the slashes so that copy/paste of the command works.
-          ml = string_replace(ml, "/", "\\")
+        if (is_clang) {
+          prefix = rebase_path("$clang_base_path/bin", root_build_dir)
+          ml = "${clang_prefix}${prefix}/${clang_cl} --target=arm64-windows"
+          if (host_os == "win") {
+            # Flip the slashes so that copy/paste of the command works.
+            ml = string_replace(ml, "/", "\\")
+          }
+          ml += " -c -o{{output}}"
+          is_msvc_assembler = false
+        } else {
+          # Only affects Arm builds with is_clang = false, implemented for building
+          # V8 for Windows on Arm systems with the MSVC toolchain.
+          ml = "armasm64.exe"
         }
-        ml += " -c -o{{output}}"
       } else {
+        # x86/x64 builds always use the MSVC assembler.
         if (toolchain_args.current_cpu == "x64") {
           ml = "ml64.exe"
         } else {
           ml = "ml.exe"
         }
-        ml += " /nologo /c /Fo{{output}}"
+      }
+
+      if (is_msvc_assembler) {
+        ml += " /nologo /Fo{{output}}"
+
+        # Suppress final-stage linking on x64/x86 builds. (Armasm64 does not
+        # require /c because it doesn't support linking.)
+        if (toolchain_args.current_cpu != "arm64") {
+          ml += " /c"
+        }
         if (use_lld) {
           # Wrap ml(64).exe with a script that makes its output deterministic.
           # It's lld only because the script zaps obj Timestamp which
@@ -247,7 +266,15 @@ template("msvc_toolchain") {
           ml = "$python_path $ml_py $ml"
         }
       }
-      command = "$python_path $tool_wrapper_path asm-wrapper $env $ml {{defines}} {{include_dirs}} {{asmflags}} {{source}}"
+      if (toolchain_args.current_cpu != "arm64" || is_clang) {
+        command = "$python_path $tool_wrapper_path asm-wrapper $env $ml {{defines}} {{include_dirs}} {{asmflags}} {{source}}"
+      } else {
+        # armasm64.exe does not support definitions passed via the command line.
+        # (Fortunately, they're not needed for compiling the V8 snapshot, which
+        # is the only time this assembler is required.)
+        command = "$python_path $tool_wrapper_path asm-wrapper $env $ml {{include_dirs}} {{asmflags}} {{source}}"
+      }
+
       description = "ASM {{output}}"
       outputs = [ "$object_subdir/{{source_name_part}}.obj" ]
     }