When a local shared library should be loaded, resolve its full path
before calling dlopen.

This logic matches SwiftShader's library loading.

This is a speculative fix for Chrome being unable to load ANGLE libraries
when they are distributed as part of a Chromium.app and have a different
path than the executable.

BUG=982294

Change-Id: Ib70096eac7460417ea7ea32941e2273dd368e6ac
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1724910
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Clemen Deng <clemendeng@google.com>