Add support for sysroots to GN's pkg-config.

This adds support similar to the GYP scripts for running pkg-config in the presence of sysroots.

It also adds the linker flags by calling the existing horrible sysroot_ld_path.sh shell script that extracts stuff from the sysroot's configuration.

With this change, official 32-bit linux builds work in the checked in 32-bit Debian sysroots.

R=piman@chromium.org

Review URL: https://codereview.chromium.org/116803003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241397 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241397