bpftool: __bpf_fastcall for kfuncs marked with special decl_tag

bpftool: __bpf_fastcall for kfuncs marked with special decl_tag

Generate __attribute__((bpf_fastcall)) for kfuncs marked with
"bpf_fastcall" decl tag. E.g. for the following BTF:

    $ bpftool btf dump file vmlinux
    ...
    [A] FUNC 'bpf_rdonly_cast' type_id=...
    ...
    [B] DECL_TAG 'bpf_kfunc' type_id=A component_idx=-1
    [C] DECL_TAG 'bpf_fastcall' type_id=A component_idx=-1

Generate the following vmlinux.h:

    #ifndef __VMLINUX_H__
    #define __VMLINUX_H__
    ...
    #ifndef __bpf_fastcall
    #if __has_attribute(bpf_fastcall)
    #define __bpf_fastcall __attribute__((bpf_fastcall))
    #else
    #define __bpf_fastcall
    #endif
    #endif
    ...
    __bpf_fastcall extern void *bpf_rdonly_cast(...) ...;

The "bpf_fastcall" / "bpf_kfunc" tags pair would generated by pahole
when constructing vmlinux BTF.

While at it, sort printed kfuncs by name for better vmlinux.h
stability.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240916091712.2929279-5-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>