bpftool: Add ability to specify custom skeleton object name

Add optional name OBJECT_NAME parameter to `gen skeleton` command to override
default object name, normally derived from input file name. This allows much
more flexibility during build time.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210318194036.3521577-9-andrii@kernel.org