Infer tvOS/watchOS archs from the CPU parameters, similarly to iOS (#2707)

iOS currently has some special logic in `transition_support.bzl` that
defaults the architecture to the value of `//command_line_option:cpu` if
the value is iOS related (or a arm64 mac), but the same was not being
done for other platforms. This was causing some confusion on our team as
although on one end the regular iOS builds were being automatically
resolved to arm64 simulator builds, on the other end everything else was
resolving to x86_64 despite no changes in the environment.

This PR adds this same special logic to tvOS/watchOS for make them
consistent with iOS. So in my case, building tvOS from a arm64 mac would
now result in a tvOS arm64 simulator build.

It's unclear to me if the iOS case was done just to support some legacy
use-case (meaning we should probably not do this for the other
platforms), so if that's the case, let me know how to best solve this!

Author: rockbruno

apple/internal/transition_support.bzl

@@ -245,13 +245,23 @@ def _cpu_string(*, environment_arch, platform_type, settings = {}):
         tvos_cpus = settings["//command_line_option:tvos_cpus"]
         if tvos_cpus:
             return "tvos_{}".format(tvos_cpus[0])
+        cpu_value = settings["//command_line_option:cpu"]
+        if cpu_value.startswith("tvos_"):
+            return cpu_value
+        if cpu_value == "darwin_arm64":
+            return "tvos_sim_arm64"
         return "tvos_x86_64"
     if platform_type == "watchos":
         if environment_arch:
             return "watchos_{}".format(environment_arch)
         watchos_cpus = settings["//command_line_option:watchos_cpus"]
         if watchos_cpus:
             return "watchos_{}".format(watchos_cpus[0])
+        cpu_value = settings["//command_line_option:cpu"]
+        if cpu_value.startswith("watchos_"):
+            return cpu_value
+        if cpu_value == "darwin_arm64":
+            return "watchos_arm64"
         return "watchos_x86_64"
 
     fail("ERROR: Unknown platform type: {}".format(platform_type))