rts/RtsSymbols: Add AArch64 outline atomic operations

Fixes #22012 by adding the symbols described in
https://github.com/llvm/llvm-project/blob/main/llvm/docs/Atomics.rst#libcalls-atomic.

Ultimately this would be better addressed by #22011, but this is a first
step in the right direction and fixes the immediate symptom.

Note that we dropped the `__arch64_cas16` operations as these provided
by all platforms's compilers. Also, we don't link directly against the
libgcc/compiler-rt definitions but rather provide our own wrappers to
work around broken toolchains (e.g. https://bugs.gentoo.org/868018).

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