Remove the symupload($host_toolchain) alias from gn_all on mac x64

The alias is only present in breakpad's GN files for an arm64 host, but
gn_all was adding it as a dependency unconditionally. It was assumed
that we only cross build arm64->x64 in official builds but perf bots
can cross build the other way. They do not upload symbols so they don't
need symupload at all, and they won't be looking for it in the host
toolchain's directory specifically. So remove the dependency on mac x64
hosts.

Fixed: 350994510
Change-Id: If15b7029358f5b57b6db807de78eefa215755bee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5678032
Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1323397}

BUILD.gn

@@ -682,7 +682,7 @@ group("gn_all") {
       # name to resolve.
       deps += [ "//third_party/breakpad:symupload" ]
     }
-    if (is_mac) {
+    if (is_mac && host_cpu == "arm64") {
       # TODO(crbug.com/349268750): This alias exists because the official Mac
       # arm64 builders (which cross-build the x64 binaries) use the hardcoded
       # path of the `$host_toolchain/symupload` to build and run the symupload