[gdb/testsuite] Fix gdb.base/corefile.exp with glibc 2.35

[gdb/testsuite] Fix gdb.base/corefile.exp with glibc 2.35

On Ubuntu 22.04.1 x86_64 (with glibc 2.35), I run into:
...
(gdb) PASS: gdb.base/corefile.exp: $_exitcode is void
bt^M
 #0  __pthread_kill_implementation (...) at ./nptl/pthread_kill.c:44^M
 #1  __pthread_kill_internal (...) at ./nptl/pthread_kill.c:78^M
 #2  __GI___pthread_kill (...) at ./nptl/pthread_kill.c:89^M
 #3  0x00007f4985e1a476 in __GI_raise (...) at ../sysdeps/posix/raise.c:26^M
 #4  0x00007f4985e007f3 in __GI_abort () at ./stdlib/abort.c:79^M
 #5  0x0000556b4ea4b504 in func2 () at gdb.base/coremaker.c:153^M
 #6  0x0000556b4ea4b516 in func1 () at gdb.base/coremaker.c:159^M
 #7  0x0000556b4ea4b578 in main (...) at gdb.base/coremaker.c:171^M
(gdb) PASS: gdb.base/corefile.exp: backtrace
up^M
 #1  __pthread_kill_internal (...) at ./nptl/pthread_kill.c:78^M
 78      in ./nptl/pthread_kill.c^M
(gdb) FAIL: gdb.base/corefile.exp: up
...

The problem is that the regexp used here:
...
gdb_test "up" "#\[0-9\]* *\[0-9xa-fH'\]* in .* \\(.*\\).*" "up"
...
does not fit the __pthread_kill_internal line which lacks the instruction
address due to inlining.

Fix this by making the regexp less strict.

Tested on x86_64-linux.