[gdb/testsuite] Fix regexp in vgdb_start

[gdb/testsuite] Fix regexp in vgdb_start

On Fedora 39 aarch64 I run into:
...
(gdb) target remote | vgdb --wait=2 --max-invoke-ms=2500 --pid=2114437^M
Remote debugging using | vgdb --wait=2 --max-invoke-ms=2500 --pid=2114437^M
relaying data between gdb and process 2114437^M
warning: remote target does not support file transfer, \
  attempting to access files from local filesystem.^M
Reading symbols from /lib/ld-linux-aarch64.so.1...^M
_start () at ../sysdeps/aarch64/dl-start.S:22^M
warning: 22     ../sysdeps/aarch64/dl-start.S: No such file or directory^M
(gdb) FAIL: gdb.base/valgrind-infcall.exp: target remote for vgdb
...

For contrast, on openSUSE Leap 15.4 x86_64 I have:
...
(gdb) target remote | vgdb --wait=2 --max-invoke-ms=2500 --pid=18797^M
Remote debugging using | vgdb --wait=2 --max-invoke-ms=2500 --pid=18797^M
relaying data between gdb and process 18797^M
warning: remote target does not support file transfer, \
  attempting to access files from local filesystem.^M
Reading symbols from /lib64/ld-linux-x86-64.so.2...^M
(No debugging symbols found in /lib64/ld-linux-x86-64.so.2)^M
0x0000000004002550 in _start () from /lib64/ld-linux-x86-64.so.2^M
(gdb) PASS: gdb.base/valgrind-infcall.exp: target remote for vgdb
...

The fail happens in vgdb_start because the regexp only matches the
"in _start ()" variant, not the "_start () at":
...
       gdb_test "$vgdbcmd" " in \\.?_start .*" "target remote for vgdb"
...
Which variant you get is determined by presence of debug info.

Fix this by also matching the "_start () at" variant.

Tested aarch64-linux and x86_64-linux.

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