GN: build minidump_stackwalk for host only

This change is fixing gn gen error for Linux
in case of MIPS:
ERROR Unresolved dependencies.
//:blink_tests(//build/toolchain/linux:mipsel)
  needs //breakpad:minidump_stackwalk(//build/toolchain/linux:mipsel)

TEST=MIPS:
$ gn gen out-gn/mips-linux --args="is_debug=false target_os=\"linux\" target_cpu=\"mipsel\" is_clang=false"
$ ninja -C out-gn/mips-linux :blink_tests

x86_64:
$ gn gen out-gn/Debug
$ ninja -C out-gn/Debug :blink_tests

Review URL: https://codereview.chromium.org/1156103004

Cr-Commit-Position: refs/heads/master@{#331449}
diff --git a/BUILD.gn b/BUILD.gn
index 48342e50..cdeaa46 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -752,7 +752,7 @@
     }
 
     if (!is_win && !is_android) {
-      deps += [ "//breakpad:minidump_stackwalk" ]
+      deps += [ "//breakpad:minidump_stackwalk($host_toolchain)" ]
     }
 
     if (is_mac) {