[gdb/testsuite] Fix clean_restart in gdb.opencl/callfuncs.exp

[gdb/testsuite] Fix clean_restart in gdb.opencl/callfuncs.exp

In test-case gdb.opencl/callfuncs.exp I noticed:
...
clean_restart [standard_testfile $testfile]
...

This doesn't have the desired effect of starting gdb with $testfile because
standard_testfile doesn't return anything.

Fix this by using "clean_restart $testfile".

While we're at it:
- move standard_testfile to the start of the file
- drop the redundant 'set testfile "callfuncs"'
- use standard_testfile .cl to properly define $srcfile
- use $srcfile instead of $testfile.cl

Tested on x86_64-linux, using the demonstrator patch in PR testsuite/33363.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33363