Right now when custom sysroot is provided as gn parameter (target_sysroot)
only 32 bit custom sysroot will be used. 64 bit sysroot (which is also needed
by 32 bit build by - for example - protobuf) is expected to be found in default
location. This change adds possibility to provide path to directory containing
both sysroots for linux (32 bit and 64 bit).

BUG=622616

Review-Url: https://codereview.chromium.org/2096323002
Cr-Commit-Position: refs/heads/master@{#402742}