bpf, selftests: Add tests for ctx access in sock_ops with single register

To verify fix ("bpf: sock_ops ctx access may stomp registers in corner case")
we want to force compiler to generate the following code when accessing a
field with BPF_TCP_SOCK_GET_COMMON,

     r1 = *(u32 *)(r1 + 96) // r1 is skops ptr

Rather than depend on clang to do this we add the test with inline asm to
the tcpbpf test. This saves us from having to create another runner and
ensures that if we break this again test_tcpbpf will crash.

With above code we get the xlated code,

  11: (7b) *(u64 *)(r1 +32) = r9
  12: (61) r9 = *(u32 *)(r1 +28)
  13: (15) if r9 == 0x0 goto pc+4
  14: (79) r9 = *(u64 *)(r1 +32)
  15: (79) r1 = *(u64 *)(r1 +0)
  16: (61) r1 = *(u32 *)(r1 +2348)
  17: (05) goto pc+1
  18: (79) r9 = *(u64 *)(r1 +32)

We also add the normal case where src_reg != dst_reg so we can compare
code generation easily from llvm-objdump and ensure that case continues
to work correctly. The normal code is xlated to,

  20: (b7) r1 = 0
  21: (61) r1 = *(u32 *)(r3 +28)
  22: (15) if r1 == 0x0 goto pc+2
  23: (79) r1 = *(u64 *)(r3 +0)
  24: (61) r1 = *(u32 *)(r1 +2348)

Where the temp variable is not used.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/159718351457.4728.3295119261717842496.stgit@john-Precision-5820-Tower

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