bpf: Support inlining bpf_kptr_xchg() helper

The motivation of inlining bpf_kptr_xchg() comes from the performance
profiling of bpf memory allocator benchmark. The benchmark uses
bpf_kptr_xchg() to stash the allocated objects and to pop the stashed
objects for free. After inling bpf_kptr_xchg(), the performance for
object free on 8-CPUs VM increases about 2%~10%. The inline also has
downside: both the kasan and kcsan checks on the pointer will be
unavailable.

bpf_kptr_xchg() can be inlined by converting the calling of
bpf_kptr_xchg() into an atomic_xchg() instruction. But the conversion
depends on two conditions:
1) JIT backend supports atomic_xchg() on pointer-sized word
2) For the specific arch, the implementation of xchg is the same as
   atomic_xchg() on pointer-sized words.

It seems most 64-bit JIT backends satisfies these two conditions. But
as a precaution, defining a weak function bpf_jit_supports_ptr_xchg()
to state whether such conversion is safe and only supporting inline for
64-bit host.

For x86-64, it supports BPF_XCHG atomic operation and both xchg() and
atomic_xchg() use arch_xchg() to implement the exchange, so enabling the
inline of bpf_kptr_xchg() on x86-64 first.

Reviewed-by: Eduard Zingerman <[email protected]>
Signed-off-by: Hou Tao <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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