[gdb/testsuite] Fix gdb.base/step-over-syscall.exp with -m32

[gdb/testsuite] Fix gdb.base/step-over-syscall.exp with -m32

When executing test-case gdb.base/step-over-syscall.exp with target board
unix/-m32, we run into:
...
(gdb) x/2i $pc^M
=> 0xf7fd5155 <__kernel_vsyscall+5>:    sysenter ^M
   0xf7fd5157 <__kernel_vsyscall+7>:    int    $0x80^M
(gdb) PASS: gdb.base/step-over-syscall.exp: fork: displaced=off: \
  pc before/after syscall instruction
stepi^M
[Detaching after fork from child process 23593]^M
0xf7fd5159 in __kernel_vsyscall ()^M
1: x/i $pc^M
=> 0xf7fd5159 <__kernel_vsyscall+9>:    pop    %ebp^M
(gdb) PASS: gdb.base/step-over-syscall.exp: fork: displaced=off: stepi fork insn
print /x $pc^M
$2 = 0xf7fd5159^M
(gdb) PASS: gdb.base/step-over-syscall.exp: fork: displaced=off: pc after stepi
FAIL: gdb.base/step-over-syscall.exp: fork: displaced=off: \
  pc after stepi matches insn addr after syscall
...

The test tries to verify that after doing a stepi at a syscall insn, the $pc
is matching the insn after the syscall insn.

However, in the case that the syscall insn is "sysenter", the stepi will land
further away, so in this case:
...
   0xf7fd5155 <__kernel_vsyscall+5>:    sysenter ^M
   0xf7fd5157 <__kernel_vsyscall+7>:    int    $0x80^M
   0xf7fd5159 <__kernel_vsyscall+9>:    pop    %ebp^M
...
the stepi will land at 0xf7fd5159 instead of 0xf7fd5157.

Fix this by detecting the sysenter/int sequence and adjusting the expected
pc.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

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

	* gdb.base/step-over-syscall.exp: Detect and handle sysenter/int
	sequence.