bench: compatibility for Bison <= 2.7

bench: compatibility for Bison <= 2.7

There used to be a bug in some skeletons, which caused the expansion of
'yylval' and 'yylloc', generating these errors:

input.cc:547:16: error: expected ',' or '...' before '(' token
 #define yylval (yystackp->yyval)
                ^
input.yy:29:39: note: in expansion of macro 'yylval'
 int yylex (yy::parser::semantic_type *yylval)
                                       ^

This bug is fixed by 'skel: better aliasing of identifiers', but a workaround
is useful when benchmarking against older versions of Bison, which are still
affected by the bug.

* etc/bench.pl.in: Rename yylval to yylvalp and yylloc to yyllocp in base
grammar 'list'.