genksyms: fix last 3 shift/reduce conflicts

The genksyms parser has ambiguities in its grammar, which are currently
suppressed by a workaround in scripts/genksyms/Makefile.

Building genksyms with W=1 generates the following warnings:

YACC scripts/genksyms/parse.tab.[ch]
scripts/genksyms/parse.y: warning: 3 shift/reduce conflicts [-Wconflicts-sr]
scripts/genksyms/parse.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples

The ambiguity arises when decl_specifier_seq is followed by '(' because
the following two interpretations are possible:

- decl_specifier_seq direct_abstract_declarator '(' parameter_declaration_clause ')'
- decl_specifier_seq '(' abstract_declarator ')'

This issue occurs because the current parser allows an empty string to
be reduced to direct_abstract_declarator, which is incorrect.

K&R [1] explains the correct grammar:

<parameter-declaration> ::= {<declaration-specifier>}+ <declarator>
| {<declaration-specifier>}+ <abstract-declarator>
| {<declaration-specifier>}+

<abstract-declarator> ::= <pointer>
| <pointer> <direct-abstract-declarator>
| <direct-abstract-declarator>

<direct-abstract-declarator> ::= ( <abstract-declarator> )
| {<direct-abstract-declarator>}? [ {<constant-expression>}? ]
| {<direct-abstract-declarator>}? ( {<parameter-type-list>}? )

This commit resolves all remaining conflicts.

We need to consider the difference between the following two examples:

[Example 1] ( <abstract-declarator> ) can become <direct-abstract-declarator>

void my_func(int (foo));

... is equivalent to:

void my_func(int foo);

[Example 2] ( <parameter-type-list> ) can become <direct-abstract-declarator>

typedef int foo;
void my_func(int (foo));

... is equivalent to:

void my_func(int (*callback)(int));

Please note that the function declaration is identical in both examples,
but the preceding typedef creates the distinction. I introduced a new
term, open_paren, to enable the type lookup immediately after the '('
token. Without this, we cannot distinguish between [Example 1] and
[Example 2].

[1]: https://cs.wmich.edu/~gupta/teaching/cs4850/sumII06/The%20syntax%20of%20C%20in%20Backus-Naur%20form.htm

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Nicolas Schier <n.schier@avm.de>