gdb/testsuite: fix "up to main" in gdb.base/corefile-exec-context.exp

On ubuntu systems with libc debug info available (libc6-dbg), I see the
following failures for the gdb.base/corefile-exec-context.exp testcase:

    show args
    Argument list to give program being debugged when it is started is "aaaaa bbbbb ccccc ddddd e\ e\ e\ e\ e".
    (gdb) PASS: gdb.base/corefile-exec-context.exp: show args
    up
    #1  __pthread_kill_internal (signo=6, threadid=133859295332160) at ./nptl/pthread_kill.c:78
    78	in ./nptl/pthread_kill.c
    (gdb) FAIL: gdb.base/corefile-exec-context.exp: move up to main

This failures is because the pattern used to parse the output of `up`
is not expecting what is seen when debugging information is present for
those frames.

This patch adjusts the pattern to allow both possible outputs.

Tested on ubuntu-22.04 and ubuntu24.04 with libc6-dbg installed for gdb
build with --with-separate-debug-dir=/usr/lib/debug.

Change-Id: I217d4b20006d0ecdb4b7a71eeb8d01597ec5ac63
Approved-By: Tom Tromey <tom@tromey.com>