Fix gn installer/linux/BUILD.gn with target_arch=arm

Widevine is only available for x86 and x64 on linux.  This
check was mistakenly removed in:
https://codereview.chromium.org/2395673002

The widevine shared libraries are not buildable for
other architectures currently (see
//third_party/widevine/cdm/BUILD.gn).

TEST=gn gen with target_cpu=arm and enable_linux_installer = true
BUG=653900

Review-Url: https://codereview.chromium.org/2401033003
Cr-Commit-Position: refs/heads/master@{#424198}
1 file changed