get_concurrent_links.py: give more RAM per job in LTO builds.

This is required for launching Control Flow Integrity on Linux x86-64,
as it uses LTO builds, which use significantly more memory during
link phase.

Failing to do that lead me to see this OOM error message on the bot:
https://chromegw.corp.google.com/i/official.desktop/builders/precise64/builds/253

BUG=565162,464797

Review URL: https://codereview.chromium.org/1492843006

Cr-Commit-Position: refs/heads/master@{#363137}
3 files changed