Merge branch 'bpftool: improve error handing for missing .BTF section'

Changbin Du says:

====================
Display error message for missing ".BTF" section and clean up empty
vmlinux.h file.

v3:
 - fix typo and make error message consistent. (Andrii Nakryiko)
 - split out perf change.
v2:
 - remove vmlinux specific error info.
 - use builtin target .DELETE_ON_ERROR: to delete empty vmlinux.h
====================

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>