[gdb/testsuite] Fix gdb.arch/amd64-stap-three-arg-disp.S

[gdb/testsuite] Fix gdb.arch/amd64-stap-three-arg-disp.S

On SLE-11 I ran into:
...
(gdb) print $_probe_arg0^M
Cannot access memory at address 0x8000003fe05c^M
(gdb) FAIL: gdb.arch/amd64-stap-special-operands.exp: probe: three_arg: \
  print $_probe_arg0
...

The memory cannot be accessed because the address used to evaluate
$_probe_arg0 at the probe point is incorrect.

The address is calculated using this expression:
...
.asciz "-4@-4(%rbp,%ebx,0)"
...
which uses $ebx, but $ebx is uninitialized at the probe point.

The test-case does contain a "movl $0, %ebx" insn to set $ebx to 0, but that
insn is placed after the probe point.  We could fix this by moving the insn
to before the probe point.  But, $ebx is also a callee-save register, so
normally, if we modify it, we also need to save and restore it, which is
currently not done.  This is currently not harmful, because we don't run the
test-case further than the probe point, but it's bound to cause confusion.

So, fix this instead by using $eax instead in the expression, and moving the
insn setting $eax to 0 to before the probe point.

gdb/testsuite/ChangeLog:

2021-01-11  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26968
	* gdb.arch/amd64-stap-three-arg-disp.S: Remove insn modifying $ebx.
	Move insn setting $eax to before probe point.