[AMDGPU] Corrected directive to use for ELF weak refs

WeakRefDirective should specify a directive to declare "a global as being a weak undefined symbol".
The directive used by AMDGPU was incorrect - ".weakref" was intended for other purposes.
The correct directive is ".weak" and it is already defined as default for ELF.
So the redefinition was removed.

Reviewers: arsenm, rampitec

Differential Revision: https://reviews.llvm.org/D87762