Implement arflags in the GN build.

This flag was recently added in GN. This patch hooks it up to the toolchains
and moves some flags to use it that previously had to be hardcoded on the
tool command itself.

Remove concurrent_links variable from gcc_toolchain which was unused.

BUG=598599

Review URL: https://codereview.chromium.org/1896163003

Cr-Commit-Position: refs/heads/master@{#388554}
5 files changed