gdb/testsuite: fix failure in gdb.base/step-over-no-symbols.exp

gdb/testsuite: fix failure in gdb.base/step-over-no-symbols.exp

This test fails on my machine:

    p /x $pc^M
    $2 = 0x55555555514e^M
    (gdb) PASS: gdb.base/step-over-no-symbols.exp: displaced=off: get after PC
    FAIL: gdb.base/step-over-no-symbols.exp: displaced=off: advanced

This is due to the check added in 5f0e2eb79e6b ("GDB/testsuite: Fix a
catastrophic step-over-no-symbols.exp failure"), that makes sure the PC
values are integer.  As documented in the TCL doc [1], "string is
integer" returns 1 if the string is a valid 32-bit integer format.  The
PC values are greater than 32 bits, so are not recognized as integers by
that test.

    % string is integer -strict 0x55555555
    1
    % string is integer -strict 0x555555555
    0

Replace the "string is integer" test with a regexp one, that verifies
the PC is a hex value.

[1] https://www.tcl.tk/man/tcl/TclCmd/string.htm#M21

gdb/testsuite/ChangeLog:

	* gdb.base/step-over-no-symbols.exp (test_step_over): Replace
	integer format test with regexp.

Change-Id: I71f8197e7b52e97b4901980544a8d1072aabd362