genksyms: fix typeof() handling

Recent increased use of typeof() throughout the tree resulted in a
number of symbols (25 in a typical distro config of ours) not getting a
proper CRC calculated for them anymore, due to the parser in genksyms
not coping with several of these uses (interestingly in the majority of
[if not all] cases the problem is due to the use of typeof() in code
preceding a certain export, not in the declaration/definition of the
exported function/object itself; I wasn't able to find a way to address
this more general parser shortcoming).

The use of parameter_declaration is a little more relaxed than would be
ideal (permitting not just a bare type specification, but also one with
identifier), but since the same code is being passed through an actual
compiler, there's no apparent risk of allowing through any broken code.

Otoh using parameter_declaration instead of the ad hoc
"decl_specifier_seq '*'" / "decl_specifier_seq" pair allows all types to
be handled rather than just plain ones an...