[macOS] Fix dynamic library lookup for system libraries.

[bruvzg]

System libraries that were under /usr/lib do not exist as physical files, but still loadable with dlopen.

New in macOS Big Sur 11.0.1, the system ships with a built-in dynamic linker cache of all system-provided libraries. As part of this change, copies of dynamic libraries are no longer present on the filesystem. Code that attempts to check for dynamic library presence by looking for a file at a path or enumerating a directory will fail. Instead, check for library presence by attempting to dlopen() the path, which will correctly check for the library in the cache. (62986286)

[akien-mga]

Thanks!