[gdb/testsuite] Allow nodebug srcfile in gdb.base/unwind-on-each-insn.exp

[gdb/testsuite] Allow nodebug srcfile in gdb.base/unwind-on-each-insn.exp

Test-case gdb.base/unwind-on-each-insn.exp compiles $srcfile with debug info, and
$srcfile2 without.

Occasionally, I try both files with debug info:
...
-             $srcfile $debug_flags $srcfile2 $nodebug_flags]]} {
+             $srcfile $debug_flags $srcfile2 $debug_flags]]} {
...
and both files without:
...
-             $srcfile $debug_flags $srcfile2 $nodebug_flags]]} {
+             $srcfile $nodebug_flags $srcfile2 $nodebug_flags]]} {
...

In the latter case, I run into:
...
FAIL: gdb.base/unwind-on-each-insn.exp: foo: instruction 1: bt 2
FAIL: gdb.base/unwind-on-each-insn.exp: foo: instruction 1: up
...
due to a mismatch between the regexp and the different output due to using
nodebug.

Fix this by making the regexp less strict.

Tested on x86_64-linux.