[libomptarget][amdgpu] Mark alloc, free weak to facilitate local experimentation

[libomptarget][amdgpu] Mark alloc, free weak to facilitate local experimentation

There are a lot of different ways we might implement the devicertl local alloc
and free functions. Via host, local buffers (stack or arena), specialising per
kernel etc. It is not yet clear what the right design is. This change makes the
alloc and free functions weak, so one can override them from local tests while
comparing options.

Not strictly necessary, as a comparable patch can be applied locally each time,
but would be convenient for out of tree dev. Plan would be to drop the weak
attribute at the same time as introducing a working allocator to trunk.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D102499