sysroot installer: Don't error out for unknown target archs

V8 has several target_archs that don't have a supported
sysroot image, e.g. mips (big endian), ppc, s390.

This prevents the script from failing when run as a hook and
when one of these architectures is used.

BUG=chromium:616032

Review-Url: https://codereview.chromium.org/2027873004
Cr-Commit-Position: refs/heads/master@{#397129}
1 file changed