Use sysroot by default for all linux builds

Using a sysroot was already the default for official
build and when cross compiling (e.g. arm, mips, x86-32).
This change makes it the default in all cases.

The old behavior of building against local
system libraries can still be achieved by
setting use_sysroot=0

BUG=504446

Review URL: https://codereview.chromium.org/1464983003

Cr-Commit-Position: refs/heads/master@{#361197}