Fix android generate_linker_map check.

This change fixes gn refusing to build when `generate_linker_map` is
true for android targets from a linux host.

Bug: 986842
Change-Id: Ieb5e4d236ca54c87cf5f09fe26ce82a9bfe3191f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715024
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Nina Satragno <nsatragno@chromium.org>
Auto-Submit: Nina Satragno <nsatragno@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#681105}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a2cb14b2badc836a161f16eda1ad72bf1d020887

toolchain/toolchain.gni

@@ -30,7 +30,7 @@ if (generate_linker_map) {
   assert(
       is_official_build,
       "Linker map files should only be generated when is_official_build = true")
-  assert(current_os == "android" || target_os == "linux",
+  assert(target_os == "android" || target_os == "linux",
          "Linker map files should only be generated for Android and Linux")
 }