tools/bpftool: Generalize BPF skeleton support and generate vmlinux.h

Adapt Makefile to support BPF skeleton generation beyond single profiler.bpf.c
case. Also add vmlinux.h generation and switch profiler.bpf.c to use it.

clang-bpf-global-var feature is extended and renamed to clang-bpf-co-re to
check for support of preserve_access_index attribute, which, together with BTF
for global variables, is the minimum requirement for modern BPF programs.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200619231703.738941-7-andriin@fb.com