[gdb/testsuite] Fix gdb.opt/solib-intra-step.exp with -m32

When running test-case gdb.opt/solib-intra-step.exp with target board
unix/-m32, we run into:
...
(gdb) step^M
__x86.get_pc_thunk.bx () at ../sysdeps/i386/crti.S:66^M
66      ../sysdeps/i386/crti.S: No such file or directory.^M
(gdb) FAIL: gdb.opt/solib-intra-step.exp: first-hit (optimized)
...

The thunk is a helper function for PIC, and given that we have line info for
it, we step into.

Fix this by allowing the step into the thunk, and stepping out of it.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-01-26  Tom de Vries  <tdevries@suse.de>

	* gdb.opt/solib-intra-step.exp: Handle stepping into thunk.