genksyms: fix syntax error for attribute before abstract_declarator

A longstanding issue with genksyms is that it has hidden syntax errors.

When a syntax error occurs, yyerror() is called. However,
error_with_pos() is a no-op unless the -w option is provided.

You can observe syntax errors by manually passing the -w option.

For example, with CONFIG_MODVERSIONS=y on v6.13-rc1:

    $ make -s KCFLAGS=-D__GENKSYMS__ init/main.i
    $ cat init/main.i | scripts/genksyms/genksyms -w
        [ snip ]
    ./include/linux/efi.h:1225: syntax error

The syntax error occurs in the following code in include/linux/efi.h:

    efi_status_t
    efi_call_acpi_prm_handler(efi_status_t (__efiapi *handler_addr)(u64, void *),
                              u64 param_buffer_addr, void *context);

The issue arises from __efiapi, which is defined as either
__attribute__((ms_abi)) or __attribute__((regparm(0))).

This commit allows abstract_declarator to be prefixed with attributes.

To avoid conflicts, I tweaked the rule for decl_specifier_seq. Due to
this change, a standalone attribute cannot become decl_specifier_seq.
Otherwise, I do not know how to resolve the conflicts.

The following code, which was previously accepted by genksyms, will now
result in a syntax error:

    void my_func(__attribute__((unused))x);

I do not think it is a big deal because GCC also fails to parse it.

    $ echo 'void my_func(__attribute__((unused))x);' | gcc -c -x c -
    <stdin>:1:37: error: unknown type name 'x'

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