Enable BTF generation in the BPF backend

This patch changes the BPF GCC backend in order to use the DWARF debug
hooks and therefore enables the user to generate BTF debugging
information with -gbtf.  Generating BTF is crucial when compiling BPF
programs, since the CO-RE (compile-once, run-everwhere) mechanism
used by the kernel BPF loader relies on it.

Note that since in eBPF it is not possible to unwind frames due to the
restrictive nature of the target architecture, we are disabling the
generation of CFA in this target.

2021-06-28  David Faust <david.faust@oracle.com>

	* config/bpf/bpf.c (bpf_expand_prologue): Do not mark insns as
	frame related.
	(bpf_expand_epilogue): Likewise.
	* config/bpf/bpf.h (DWARF2_FRAME_INFO): Define to 0.
	Do not define DBX_DEBUGGING_INFO.