[infra] Setup for x64 reclient on Linux ARM64.

Change-Id: I2ff065c8b962d768f93ea3304544ceb527f9c266
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/429466
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

DEPS

@@ -482,6 +482,19 @@ deps = {
     'dep_type': 'cipd',
   },
 
+  # linux-arm64 package is not available, use the linux-x64 package and
+  # rely on transparent emulation.
+  Var("dart_root") + '/buildtools/reclient-linux': {
+    'packages': [
+      {
+        'package': 'infra/rbe/client/linux-amd64',
+        'version': Var('reclient_version'),
+      }
+    ],
+    'condition': 'host_os == "linux"',
+    'dep_type': 'cipd',
+  },
+
   Var("dart_root") + "/third_party/webdriver/chrome": {
     "packages": [
       {

build/toolchain/linux/BUILD.gn

@@ -22,7 +22,7 @@ if (use_rbe) {
       rewrapper_args + [ "--labels=type=compile,compiler=clang,lang=cpp" ]
 
   # TODO: Unfortunately I see no way to get build_arch reliably.
-  if (rbe_os != host_os) {
+  if (rbe_os != host_os || rbe_cpu != host_cpu) {
     compiler_args += [
       "--inputs=build/rbe,buildtools/$rbe_os-$rbe_cpu/clang/bin/llvm",
       "--remote_wrapper=../../build/rbe/llvm.sh",

build/toolchain/rbe.gni

@@ -29,6 +29,9 @@ declare_args() {
   if (host_os == "win") {
     rbe_dir =
         rebase_path(_dart_root + "/buildtools/reclient-win", root_build_dir)
+  } else if (host_os == "linux") {
+    rbe_dir =
+        rebase_path(_dart_root + "/buildtools/reclient-linux", root_build_dir)
   } else {
     rbe_dir = rebase_path(_dart_root + "/buildtools/reclient", root_build_dir)
   }

tools/build.py

@@ -44,7 +44,7 @@ def BuildOptions():
     other_group.add_argument("-j",
                              type=int,
                              help='Ninja -j option for RBE builds.',
-                             default=200 if sys.platform == 'win32' else 1000)
+                             default=200 if sys.platform == 'win32' else 500)
     other_group.add_argument("-l",
                              type=int,
                              help='Ninja -l option for RBE builds.',
@@ -134,10 +134,12 @@ bootstrap_path = None
 def StartRBE(out_dir, env):
     global rbe_started, bootstrap_path
     if not rbe_started:
-        if utils.IsWindows():
-          rbe_dir = 'buildtools/reclient-win'
+        if HOST_OS == 'win32':
+            rbe_dir = 'buildtools/reclient-win'
+        elif HOST_OS == 'linux':
+            rbe_dir = 'buildtools/reclient-linux'
         else:
-          rbe_dir = 'buildtools/reclient'
+            rbe_dir = 'buildtools/reclient'
         with open(os.path.join(out_dir, 'args.gn'), 'r') as fp:
             for line in fp:
                 if 'rbe_dir' in line:
@@ -300,6 +302,11 @@ def Main():
         env.pop('LIBRARY_PATH', None)
         env.pop('SDKROOT', None)
 
+    # Help QEMU binfmt work for executables that include dynamic links, such as
+    # reclient's scandeps_server.
+    if sys.platform == 'linux':
+        env['QEMU_LD_PREFIX'] = "/usr/x86_64-linux-gnu/"
+
     # Always run GN before building.
     gn_py.RunGnOnConfiguredConfigurations(options, env)