gdb/testsuite: Restore libc_has_debug_info's less strict behaviour

gdb/testsuite: Restore libc_has_debug_info's less strict behaviour

The code that was factored out from gdb.base/relativedebug.exp assumed that
libc has debug info and only determined that it doesn't if it saw a specific
message from GDB to that effect.  In the process of factoring it into a
require predicate, I made it stricter by trying to make a specific
determination of whether or not debug info is available.

Pedro noticed that "It'll disable the testcase on systems that link with
their libc statically (even if has debug info), or systems that name their
libc something else."  Which is something I hadn't considered.

This patch returns libc_has_debug_info to the original behaviour.

Also, remove a verbose message that is redundant with the $message
variable.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31700
Approved-By: Tom Tromey <tom@tromey.com>