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}

• BUILD.gn[diff]