On Linux-based machines, a shared library's path may be resolved in
multiple ways, such as from LD_LIBRARY_PATH. For these OSes, use dlopen
to see if it exists. Note that trying to create a vk::DynamicLoader with
an invalid path throws an exception, and enabling exceptions is not
always possible, so we want to detect the error case beforehand.

Bug: b/177624844
Change-Id: I5f27cfe485afbac0e4f6bda14eed2809669c2510
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52629Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Presubmit-Ready: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>