support frontends use vmlinux.h, and python tool use vmlinux.h

Could ClangLoader::parse, ClangLoader::do_compile, KBuildHelper::get_flags all switch to use vmlinux.h, instead of kernel headers?
And python bcc tool also use vmlinux.h which dump from /sys/kernel/btf/vmlinux ?

Locate ClangLoader::parse, ClangLoader::do_compile, KBuildHelper::get_flags, and the Python BCC tools that currently use kernel headers. Start by tracing how these entry points obtain headers and inspect the handling of /sys/kernel/btf/vmlinux. Done means the named frontends and Python tools use the dumped vmlinux.h as requested, with the relevant existing checks still passing.

Written by the indexing model from the issue text.