[gdb/testsuite] Fix list-missing-source.exp with gcc 4.8

[gdb/testsuite] Fix list-missing-source.exp with gcc 4.8

With gcc 4.8.1, we see this FAIL:
...
(gdb) PASS: gdb.base/list-missing-source.exp: list
info source^M
Current source file is outputs/gdb.base/list-missing-source/main.c^M
Source language is c.^M
Producer is GNU C 4.8.5 -mtune=generic -march=x86-64 -g -fno-stack-protector.^M
Compiled with DWARF 2 debugging format.^M
Does not include preprocessor macro info.^M
(gdb) FAIL: gdb.base/list-missing-source.exp: info source
...

The problem is that a "Compilation directory is <dir>" line is expected, but
this is missing due to the fact the the compilation unit for main.c doesn't
contain a DW_AT_comp_dir in the DW_TAG_compile_unit DIE.

Fix this by allowing the "Compilation directory" line to be missing.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-10-03  Tom de Vries  <tdevries@suse.de>

	PR testsuite/25059
	* gdb.base/list-missing-source.exp: Allowing the "Compilation
	directory" line to be missing.