gdb/testsuite: Use -J option when compiling Fortran tests

gdb/testsuite: Use -J option when compiling Fortran tests

When compiling Fortran tests (e.g. gdb.fortran/info-modules.exp), the
Fotran compile produces .mod files.  These files contain details of
compiled modules that are then consumed by the compiler when compiling
other files that USE a module.

Currently the compiler writes the .mod files into its current
directory, so for us this turns out to be 'build/gdb/testsuite/'.
This means that .mod files can be shared between tests, which seems
against the spirit of the GDB testsuite; source files should be
compiled fresh for each test.

This commit adds the -J option to the compiler flags whenever we
compile a Fortran file, this option tells the compiler where to write,
and look for, .mod files.

After this commit there was one Fortran test that needed fixing, with
that fix in place all of the Fortran tests pass again, but now the
.mod files are now produced in the per-test output directories.

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (gdb_compile): Add -J compiler option when building
	Fortran tests.
	* gdb.mi/mi-fortran-modules.exp: Compile source files in correct
	order.

Change-Id: I99444cf22d80e320093d3f3ed9abb8825f378e0b