gdb: make gdb.arch/amd64-disp-step-avx.exp actually test displaced stepping

gdb: make gdb.arch/amd64-disp-step-avx.exp actually test displaced stepping

The test gdb.arch/amd64-disp-step-avx.exp is meant to test that doing a
displaced step of an AVX instruction works correctly.  However, I found
(by pure coincidence) that the test instructions are not actually
displaced stepped.  Rather, they are inline-stepped, so the test is not
actually testing what it's meant to test.

This is what a portion of the test binary looks like:

    0000000000400180 <_start>:
      400180:       90                      nop

    0000000000400181 <main>:
      400181:       90                      nop

    0000000000400182 <test_rip_vex2>:
      400182:       c5 fb 10 05 0e 00 00    vmovsd 0xe(%rip),%xmm0        # 400198 <ro_var>
      400189:       00

    000000000040018a <test_rip_vex2_end>:
      40018a:       90                      nop

The instruction at 0x400182 is the one we want to test a displaced step
for.  A breakpoint is placed at 0x400182 and ran to.  The execution is
then resumed from there, forcing a step-over (which should normally be a
displaced step) of the breakpoint.

However, the displaced stepping buffer is at the _start label, and that
means a breakpoint is present in the displaced stepping buffer.  The
breakpoint_in_range_p check in displaced_step_prepare_throw evaluates to
true, which makes displaced_step_prepare_throw fail, forcing GDB to fall
back on an in-line step.

This can be easily observed by placing a `gdb_assert (false)` inside the
breakpoint_in_range_p condition, in displaced_step_prepare_throw, and
running gdb.arch/amd64-disp-step-avx.exp.  The assertion will make the
test fail.

The proposed fix is to pad `_start` with a bunch of nops so that the
test instruction is out of the displaced step buffer.

I also think it would be good to enhance the test to make sure that we
are testing displaced stepping as intended.  I did that by enabling "set
debug displaced on" while we step over the interesting instruction, and
matching a message printed only when a displaced step is executed.

gdb/testsuite/ChangeLog:

	* gdb.arch/amd64-disp-step-avx.S: Add nops after _start.
	* gdb.arch/amd64-disp-step-avx.exp: Enable "set debug displaced
	on" while stepping over the test instruction, match printed
	message.