[Libomptarget] Allow the CPU targets to be built without libffi (#77495)
Summary:
The CPU targets currently rely on `libffi` to invoke the "kernel"
functions. Previously we would not build these if this dependency was
not found. This patch copies th eapproach used for things like CUDA and
HSA to dynamically load this if it is not found.
The one sketchy thing this does is hard-code the default ABI for the
target. These are normally defined on a per-file basis in the FFI
source, so I had to fish out the expected values. We only use two types,
so ideally we will always be able to use the default ABI.
It's possible we could remove this dependency entirely in the future as
well.
GitOrigin-RevId: c7c68f1764ddd38d940946007c634b4bacb902b2
5 files changed
