Disable trivial weak_ptr test on ARM because it is not expected to work.

Summary:
weak_ptr has two pointers (more than the 4 bytes limit), so it will not be returned in registers on ARM, even if it is trivial.
The test, therefore, will fail on ARM.

Reviewers: #libc!

Subscribers: kristof.beyls, danielkiss, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D84200