build: bpf: fix LLVM tool paths with host toolchain

build: bpf: fix LLVM tool paths with host toolchain

Do not assume that the various tools like llc can be found under the
same path as clang; instead, look them up through BPF_PATH (while still
preferring ones found next to clang).

This fixes build in common setups with ccache, where clang resolves to a
path like /usr/lib/ccache/bin/clang, but no other tools can be found at
that location.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Link: https://github.com/openwrt/openwrt/pull/18422
Signed-off-by: Robert Marko <robimarko@gmail.com>