Fix linux OS check for generate_linker_map

This was changed by r554983, which really should have only changed the
android OS check, as Linux builds also need to support NaCl toolchains
where current_os != "linux".

Bug: 822034
Change-Id: I5ca924516158bdd6228b947f32db6b614d1caacd
Reviewed-on: https://chromium-review.googlesource.com/1038430Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555228}

build/toolchain/toolchain.gni

@@ -36,7 +36,7 @@ if (generate_linker_map) {
   assert(
       is_official_build,
       "Linker map files should only be generated when is_official_build = true")
-  assert(current_os == "android" || current_os == "linux",
+  assert(current_os == "android" || target_os == "linux",
          "Linker map files should only be generated for Android and Linux")
 }