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rts/linker: Don't fail due to RTLD_NOW

Ben Gamari requested to merge wip/T25943 into master

In !12264 (closed) we started using the NativeObj machinery introduced some time ago for loading of shared objects. One of the side-effects of this change is shared objects are now loaded eagerly (i.e. with RTLD_NOW). This is needed by NativeObj to ensure full visibility of the mappings of the loaded object, which is in turn needed for safe shared object unloading.

Unfortunately, this change subtly regressed, causing compilation failures in some programs. Specifically, shared objects which refer to undefined symbols (e.g. which may be usually provided by either the executable image or libraries loaded via dlopen) will fail to load with eager binding. This is problematic as GHC loads all package dependencies while, e.g., evaluating TemplateHaskell splices. This results in compilation failures in programs depending upon (but not using at compile-time) packages with undefined symbol references.

To mitigate this NativeObj now first attempts to load an object via eager binding, reverting to lazy binding (and disabling unloading) on failure.

See Note [Don't fail due to RTLD_NOW].

Fixes #25943 (closed).

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