[Support] Check for atomics64 when deciding if '-latomic' is needed

The CheckAtomic module performs two tests to determine if passing
'-latomic' to the linker is required: one for 64-bit atomics, and
another for non-64-bit atomics. Include the missing check for 64-bit
atomics.

Reviewers: beanz, compnerd
Reviewed By: beanz, compnerd
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D69444