Add system test before "set remote system-call-allowed 1" to fileio.exp

This patch is update version according to the discussion in
https://www.sourceware.org/ml/gdb-patches/2009-11/msg00090.html.
If test get the target doesn't support fileio system according to the
remote log.   It will set this test as "unsupported".

Before I made this patch, I want add a check before all of tests in this
file.  But I found that the target maybe support one call but not others.
For example: my target support Fwrite, Fopen and so on.  But not Fgettimeofday.
And it doesn't support Fsystem NULL but it support Fsystem not NULL.
So I think if we want to check target support fileio, we need check them
one by one.

2014-06-04  Nathan Sidwell  <nathan@codesourcery.com>
	    Hui Zhu  <hui@codesourcery.com>

	* gdb.base/fileio.exp: Add test for shell not available as well as
	available.
	* gdb.base/fileio.c (test_system): Check for shell twice.