Prepare directory in case test_system fails

In gdb.base/fileio.c, some functions may depend on others.  For
example, test_rename renames a file to one directory which is created
in test_system.  That is means, if test_system fails, test_rename
fails too, which is not a good practise, IMO.

In test_system, system ("mkdir -p XX") is used to create directories
needed for test_rename.  In this patch, we use dejagnu remote_exec
proc to create these directories on host.

In my gdb testing, mingw32 host and arm-none-eabi target, system
("mkdir -p XX") doesn't work properly (this issue can be addressed
separately), and this patch fixes the following fails.

FAIL: gdb.base/fileio.exp: Renaming a directory to a non-empty directory returns ENOTEMPTY or EEXIST
FAIL: gdb.base/fileio.exp: Unlink a file
FAIL: gdb.base/fileio.exp: Unlinking a file in a directory w/o write access returns EACCES

gdb/testsuite:

2014-10-29  Yao Qi  <yao@codesourcery.com>

	* gdb.base/fileio.exp: Make directories on host.