[gdb/testsuite] Fix valgrind tests on debian

On debian 12, I run into:
...
(gdb) target remote | vgdb --wait=2 --max-invoke-ms=2500 --pid=618591^M
Remote debugging using | vgdb --wait=2 --max-invoke-ms=2500 --pid=618591^M
relaying data between gdb and process 618591^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
(No debugging symbols found in /lib/ld-linux-aarch64.so.1)^M
0x000000000401a980 in ?? () from /lib/ld-linux-aarch64.so.1^M
(gdb) FAIL: gdb.base/valgrind-infcall.exp: target remote for vgdb
...

The problem is that we're expecting to match either of these regexps:
...
	set start_re1 " in \\.?_start "
        set start_re2 "\\.?_start \\(\\) at "
...
but there are no dwarf or elf symbols present.

Fix this by also allowing:
...
       set start_re3 "$::hex in \\?\\? \\(\\) from "
...

Tested on aarch64-linux.

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