[AArch64][Clang][Linux] Enable out-of-line atomics by default.

Generate outline atomics if compiling for armv8-a non-LSE AArch64 Linux
(including Android) targets to use LSE instructions, if they are available,
at runtime. Library support is checked by clang driver which doesn't enable
outline atomics if no proper libraries (libgcc >= 9.3.1 or compiler-rt) found.

Differential Revision: https://reviews.llvm.org/D93585