[gdb/testsuite] Fix stepi test-cases with unix/-m32/-fPIE/-pie

When running test-case gdb.base/step-indirect-call-thunk.exp with target board
unix/-m32/-fPIE/-pie, I run into:
...
(gdb) stepi^M
0x5655552e      22      {                /* inc.1 */^M
(gdb) stepi^M
0x56555530      22      {                /* inc.1 */^M
(gdb) stepi^M
0x565555f7 in __x86.get_pc_thunk.ax ()^M
(gdb) FAIL: gdb.base/step-indirect-call-thunk.exp: stepi into return thunk
...

In contrast, with unix/-m32 we have instead:
...
(gdb) stepi^M
0x08048407      22      {                /* inc.1 */^M
(gdb) stepi^M
23        return x + 1;  /* inc.2 */^M
(gdb) stepi^M
0x0804840c      23        return x + 1;  /* inc.2 */^M
(gdb) stepi^M
24      }                /* inc.3 */^M
(gdb) stepi^M
0x08048410      24      }                /* inc.3 */^M
(gdb) stepi^M
0x0804848f in __x86_return_thunk ()^M
(gdb) PASS: gdb.base/step-indirect-call-thunk.exp: stepi into return thunk
...

The test-case doesn't expect to run into __x86.get_pc_thunk.ax, which is a
PIC helper function for x86_64-linux.

Fix this by insn-stepping through it.

Likewise in a few other test-cases.

Tested on x86_64-linux.