Atomics for eBPF patch series adds support for atomic[64]_fetch_add,
atomic[64]_[fetch_]{and,or,xor} and atomic[64]_{xchg|cmpxchg}, but it
only adds support for x86-64, so support these atomic operations for
arm64 as well.

Basically the implementation procedure is almost mechanical translation
of code snippets in atomic_ll_sc.h & atomic_lse.h & cmpxchg.h located
under arch/arm64/include/asm.

When LSE atomic is unavailable, an extra temporary register is needed for
(BPF_ADD | BPF_FETCH) to save the value of src register, instead of adding
TMP_REG_4 just use BPF_REG_AX instead. Also make emit_lse_atomic() as an
empty inline function when CONFIG_ARM64_LSE_ATOMICS is disabled.

For cpus_have_cap(ARM64_HAS_LSE_ATOMICS) case and no-LSE-ATOMICS case, the
following three tests: "./test_verifier", "./test_progs -t atomic" and
"insmod ./test_bpf.ko" are exercised and passed.
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220217072232.1186625-4-houtao1@huawei.com