[gdb/testsuite] Add -lbl option in gdb_test_multiple

Add gdb_test_multiple option -lbl, that adds a regexp after the user code that
reads one line, and discards it:
...
           -re "\r\n\[^\r\n\]*(?=\r\n)" {
               exp_continue
           }
...

In order to be able to write:
...
gdb_test_multiple "command" "testname" -lbl {
  ...
}
...
rewrite the promp_regexp argument usage into the similar:
...
gdb_test_multiple "command" "testname" -prompt $prompt_regexp {
  ...
}
...

Build and reg-tested on x86_64-linux.

Tested gdb.base/corefile-buildid.exp with both make targets check and
check-read1.

gdb/testsuite/ChangeLog:

2020-03-02  Pedro Alves  <palves@redhat.com>
	      Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (gdb_test_multiple): Handle prompt_regexp option using
	-prompt prefix, before user_code argument.  Add -lbl option likewise.
	(skip_python_tests_prompt, skip_libstdcxx_probe_tests_prompt)
	(gdb_is_target_1): Add -prompt prefix and move to before user_code
	argument.
	* gdb.base/corefile-buildid.exp: Use -lbl option.  Rewrite regexps to
	have "\r\n" at start-of-line, instead of at end-of-line.