Support framework loading on macOS Big Sur

macOS Big Sur makes the following change to how frameworks are shipped with the OS:

New in macOS Big Sur 11 beta, 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)

(https://developer.apple.com/documentation/macos-release-notes/macos-big-sur-11-beta-release-notes/)

Therefore, the previous method of checking whether a library exists before attempting to load it makes GHC.Runtime.Linker.loadFramework fail to find frameworks installed at /System/Library/Frameworks. Instead, any attempt to load a framework at runtime, such as by passing -framework OpenGL to runghc or running code loading such a framework with GHCi, fails with a not found message.

GHC.Runtime.Linker.loadFramework now opportunistically loads the framework libraries without checking for their existence first, failing only if all attempts to load a given framework from any of the various possible locations fail.

Fixes #18446 (closed).