[gdb/testsuite] Fix gdb.base/fileio.exp for local-remote-host-native

[gdb/testsuite] Fix gdb.base/fileio.exp for local-remote-host-native

When running test-case gdb.base/fileio.exp with host/target board
local-remote-host-native I run into:
...
gdb compile failed, fileio.c: In function 'test_open':
<command-line>:0:8: error: expected expression before '.' token
fileio.c:89:15: note: in expansion of macro 'OUTDIR'
   ret = open (OUTDIR FILENAME, O_CREAT | O_TRUNC | O_RDWR, S_IWUSR | S_IRUSR);
               ^~~~~~
...

Fix this by using:
- quote_for_host to pass -DOUTDIR to the compilation, and
- STRINGIFY to stringify OUTDIR in fileio.c.

Tested on x86_64-linux, with target board unix and host/target board
local-remote-host-native.