[gdb/testsuite] Fix gdb.dwarf2/dw2-using-debug-str.exp with cc-with-dwz-m

[gdb/testsuite] Fix gdb.dwarf2/dw2-using-debug-str.exp with cc-with-dwz-m

When running with target board cc-with-dwz-m, we run into:
...
(gdb) file dw2-using-debug-str-no-debug-str^M
Reading symbols from dw2-using-debug-str-no-debug-str...^M
(gdb) FAIL: gdb.dwarf2/dw2-using-debug-str.exp: file dw2-using-debug-str
...

With native, the .debug_str section is present in the
dw2-using-debug-str executable, and removed from the
dw2-using-debug-str-no-debug-str executable.  When loading the latter, a dwarf
error is triggered.

With cc-with-dwz-m, the .debug_str section is not present in the
dw2-using-debug-str executable, because it's already moved to
.tmp/dw2-using-debug-str.dwz.  Consequently, the removal has no effect, and no
dwarf error is triggered, which causes the FAIL.

The same problem arises with target board cc-with-gnu-debuglink.

Fix this by detecting whether the .debug_str section is missing, and skipping
the remainder of the test-case.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-08-02  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/dw2-using-debug-str.exp: Handle missing .debug_str
	section in dw2-using-debug-str.