[gdb/testsuite] Fix gdb.arch/i386-mpx.exp with -m32

[gdb/testsuite] Fix gdb.arch/i386-mpx.exp with -m32

When running test-case gdb.arch/i386-mpx.exp with target board unix/-m32, we
run into:
...
(gdb) print $bndstatus^M
$3 = {raw = 0xf7ca7ff2, status = {bde = 1039310844, error = 2}}^M
(gdb) FAIL: gdb.arch/i386-mpx.exp: bndstatus formating
print $bndstatus.raw^M
$4 = (void *) 0xf7ca7ff2^M
(gdb) FAIL: gdb.arch/i386-mpx.exp: bndstatus is zero by startup
...

The failure does not occur with -m64, there we have instead:
...
(gdb) print $bndstatus^M
$3 = {raw = 0x0, status = {bde = 0, error = 0}}^M
(gdb) PASS: gdb.arch/i386-mpx.exp: bndstatus formating
print $bndstatus.raw^M
$4 = (void *) 0x0^M
(gdb) PASS: gdb.arch/i386-mpx.exp: bndstatus is zero by startup
...

The difference is as follows.  At the point of issuing the print commands, we
have run to main, so in the case of -m64 we have executed:
...
00000000004004c7 <main>:
  4004c7:       55                      push   %rbp
  4004c8:       48 89 e5                mov    %rsp,%rbp
  4004cb:       89 7d fc                mov    %edi,-0x4(%rbp)
  4004ce:       48 89 75 f0             mov    %rsi,-0x10(%rbp)
  4004d2:       66 0f 1b 45 e0          bndmov %bnd0,-0x20(%rbp)
...
and in the case of -m32:
...
08048426 <main>:
 8048426:       55                      push   %ebp
 8048427:       89 e5                   mov    %esp,%ebp
 8048429:       83 ec 08                sub    $0x8,%esp
 804842c:       8d 45 0c                lea    0xc(%ebp),%eax
 804842f:       8b 55 0c                mov    0xc(%ebp),%edx
 8048432:       0f 1a 04 10             bndldx (%eax,%edx,1),%bnd0
 8048436:       66 0f 1b 45 f8          bndmov %bnd0,-0x8(%ebp)
...

In both cases, the bnd instructions attempt to save the bound for pointer
argument argv to stack.  However, there's no such bound set.

In the -m64 case, that means we just save some random value to stack.

In the -m32 case, that means that when executing bndldx the corresponding
entry in the Bounds Directory is invalid, and $bndstatus is updated to reflect
that.

Fix this by dropping the unnecessary argv parameter to main, similar to all
other gdb.arch/i386-mpx*.c test-cases.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-01-19  Tom de Vries  <tdevries@suse.de>

	* gdb.arch/i386-mpx.c (main): Drop argc/argv parameter.