Make Linux GN build's cflags match GYP's

This is for the base shared library target on desktop Linux.

The remaining difference on Linux is that GYP's base build specifies -Wno-write-strings on Linux. This dates back to the first GYP file. I don't know why we would specify this for only base, and it seems not to trigger any warning without this flag, so I didn't add it.

This changes the pkg-config invocation from --libs to --libs-only-L and --libs-only-l. The output seems slightly different (--libs was producing an extra --export-dynamic for gmodule-2.0 that aren't present when invoking the other way, which is what the GYP build does).

R=thakis@chromium.org

Review URL: https://codereview.chromium.org/165873004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252409 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252409