[gdb/testsuite] Update psym-external-decl.exp for gcc-10/clang

[gdb/testsuite] Update psym-external-decl.exp for gcc-10/clang

When running test-case gdb.base/psym-external-decl.exp with gcc-10, we have:
...
(gdb) print aaa^M
'aaa' has unknown type; cast it to its declared type^M
(gdb) FAIL: gdb.base/psym-external-decl.exp: print aaa
...

With an an earlier version, gcc still emits the debug info for the
declaration of aaa:
...
 <0><d2>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <d8>   DW_AT_name        : psym-external-decl.c
 <1><f4>: Abbrev Number: 2 (DW_TAG_variable)
    <f5>   DW_AT_name        : aaa
    <ff>   DW_AT_external    : 1
    <ff>   DW_AT_declaration : 1
...
but with gcc-10 that's no longer the case.

Fix the test-case by adding a use of aaa in psym-external-decl.c.

That still doesn't work for clang, so skip test in that case.

Tested with x86_64-linux, with gcc 7.5.0, gcc 10.0.0 and clang 5.0.2.

Also tested by reverting corresponding fix and ensuring test-case still
fails.

gdb/testsuite/ChangeLog:

2020-05-02  Tom de Vries  <tdevries@suse.de>

	* gdb.base/psym-external-decl.c (main): Add use of variable aaa.