[build] Rely on default host os TARGET_OS unless cross-compiling.

Before this change we erroneously override TARGET_OS with target_os value, which for example resulted in dart sdk built as part of Fuchsia targeting Fuchsia, rather than host os(Linux).

Bug: https://bugs.fuchsia.dev/p/fuchsia/issues/detail?id=36723
Change-Id: I92b698a331c8c5ef802f0b329ebf13616f29952f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/118372
Reviewed-by: Zach Anderson <zra@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>

Author: aam

runtime/BUILD.gn

@@ -67,11 +67,9 @@ config("dart_nosnapshot_config") {
 config("dart_os_config") {
   defines = []
 
-  # If dart_host_toolchain is the current toolchain, and it is different from
-  # host_toolchain, then we are building the SDK for the host, and should not
-  # hardcode these defines.
-  if (current_toolchain != dart_host_toolchain ||
-      host_toolchain == dart_host_toolchain) {
+  # If dart_host_toolchain is the current toolchain then we are building
+  # the SDK for the host, and should not hardcode these defines.
+  if (current_toolchain != dart_host_toolchain) {
     if (target_os == "android") {
       defines += [ "TARGET_OS_ANDROID" ]
     } else if (target_os == "fuchsia") {