devel/dwarves: fix build on powerpc*

/wrkdirs/usr/ports/devel/dwarves/work/dwarves-1.31/lib/bpf/src/bpf.c:63:4: error: __NR_bpf not defined. libbpf does not support your arch.

I found __NR_bpf in Linux kernel, other architectures we use also match that macro in Linux.