This patch enables -gprune-btf by default in the BPF backend when
generating BTF information, and fixes BPF CO-RE generation when using
-gprune-btf.

When generating BPF CO-RE information, we must ensure that types used
in CO-RE relocations always have sufficient BTF information emited so
that the CO-RE relocations can be processed by a BPF loader.  The BTF
pruning algorithm on its own does not have sufficient information to
determine which types are used in a BPF CO-RE relocation, so this
information must be supplied by the BPF backend, using a new
btf_mark_type_used function.

Co-authored-by: Cupertino Miranda <cupertino.miranda@oracle.com>

gcc/
	* btfout.cc (btf_mark_type_used): New.
	* ctfc.h (btf_mark_type_used): Declare it here.
	* config/bpf/bpf.cc (bpf_option_override): Enable -gprune-btf
	by default if -gbtf is enabled.
	* config/bpf/core-builtins.cc (extra_fn): New typedef.
	(compute_field_expr): Add callback parameter, and call it if supplied.
	Fix computation for MEM_REF.
	(mark_component_type_as_used): New.
	(bpf_mark_types_as_used): Likewise.
	(bpf_expand_core_builtin): Call here.
	* doc/invoke.texi (Debugging Options): Note that -gprune-btf is
	enabled by default for BPF target when generating BTF.

gcc/testsuite/
	* gcc.dg/debug/btf/btf-variables-5.c: Adjust one test for bpf-*-*
	target.