clang: Only build arm64 iossim compiler-rt libs on the upload bots.

For reasons not yet understood, building it on the tot bots makes
the tot bots fail. It works fine on the upload bot, so only build
it there.

This change doesn't affect the clang package we distribute since it's
a no-op on the upload bots, and the tot bots don't use the arm64
iossim slice. We still should understand what's going on, since having
the tot bots and the upload bots do different things is suboptimal.

Bug: 1145151,1138425
Change-Id: Ib8d7a31289ec2caf6ccf2fe6e6ddadb4f908b4c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517240Reviewed-by: Hans Wennborg <hans@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823529}

tools/clang/scripts/build.py

@@ -784,6 +784,11 @@ def main():
     '-DCOMPILER_RT_BUILD_XRAY=OFF',
   ]
   if sys.platform == 'darwin':
+    # TODO(crbug.com/1145151): Figure out why arm64 causes build errors on
+    # the tot bots, fix that, and always build arm64 in iossim_archs.
+    iossim_archs = 'i386;x86_64'
+    if args.bootstrap:
+      iossim_archs += ';arm64'
     compiler_rt_args.extend([
         '-DCOMPILER_RT_BUILD_BUILTINS=ON',
         '-DCOMPILER_RT_ENABLE_IOS=ON',
@@ -792,7 +797,7 @@ def main():
         # armv7 is A5 and earlier, armv7s is A6+ (2012 and later, before 64-bit
         # iPhones). armv7k is Apple Watch, which we don't need.
         '-DDARWIN_ios_ARCHS=armv7;armv7s;arm64',
-        '-DDARWIN_iossim_ARCHS=i386;x86_64;arm64',
+        '-DDARWIN_iossim_ARCHS=' + iossim_archs,
     ])
     if args.bootstrap:
       # mac/arm64 needs MacOSX11.0.sdk. System Xcode (+ SDK) on the chrome bots