bpf_progs: disable BTF on <=U && user builds

This is to cut down bpfloader boot time.
Potential savings might be on the order of 30+% (300ms).

Loading BTF requires fork-execing the btfloader,
and currently BTF is only used to facilitate debugging.

Bug: 286369326
Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ifa5f0052135b9dc826b18ca4622784615ed9c3c8