Add more directives to benchmark input files

Add more directives to benchmark input files

This patch adds some more directives to the benchmark inputs file,
moving functionality from the Makefile and making the code generation
script a bit cleaner.  The function argument and return types that
were earlier added as variables in the makefile and passed to the
script via command line arguments are now the 'args' and 'ret'
directive respectively.  'args' should be a colon separated list of
argument types (skipped if the function doesn't accept any arguments)
and 'ret' should be the return type.

Additionally, an 'includes' directive may have a comma separated list
of headers to include in the source.  For example, the pow input file
now looks like this:

42.0, 42.0
1.0000000000000020, 1.5

I did this to unclutter the benchtests Makefile a bit and eventually
eliminate dependency of the tests on the Makefile and have tests
depend on their respective include files only.