btf: add -gprune-btf option

This patch adds a new option, -gprune-btf, to control BTF debug info
generation.

As the name implies, this option enables a kind of "pruning" of the BTF
information before it is emitted.  When enabled, rather than emitting
all type information translated from DWARF, only information for types
directly used in the source program is emitted.

The primary purpose of this pruning is to reduce the amount of
unnecessary BTF information emitted, especially for BPF programs.  It is
very common for BPF programs to include Linux kernel internal headers in
order to have access to kernel data structures.  However, doing so often
has the side effect of also adding type definitions for a large number
of types which are not actually used by nor relevant to the program.
In these cases, -gprune-btf commonly reduces the size of the resulting
BTF information by 10x or more, as seen on average when compiling Linux
kernel BPF selftests.  This both slims down the size of the resulting
object and reduces the time required by the BPF loader to verify the
program and its BTF information.

Note that the pruning implemented in this patch follows the same rules
as the BTF pruning performed unconditionally by LLVM's BPF backend when
generating BTF.  In particular, the main sources of pruning are:

  1) Only generate BTF for types used by variables and functions at the
     file scope.

     Note that which variables are known to be "used" may differ
     slightly between LTO and non-LTO builds due to optimizations.  For
     non-LTO builds (and always for the BPF target), variables which are
     optimized away during compilation are considered to be unused, and
     they (along with their types) are pruned.  For LTO builds, such
     variables are not known to be optimized away by the time pruning
     occurs, so VAR records for them and information for their types may
     be present in the emitted BTF information.  This is a missed
     optimization that may be fixed in the future.

  2) Avoid emitting full BTF for struct and union types which are only
     pointed-to by members of other struct/union types.  In these cases,
     the full BTF_KIND_STRUCT or BTF_KIND_UNION which would normally
     be emitted is replaced with a BTF_KIND_FWD, as though the
     underlying type was a forward-declared struct or union type.

gcc/
	* btfout.cc (btf_used_types): New hash set.
	(struct btf_fixup): New.
	(fixups, forwards): New vecs.
	(btf_output): Calculate num_types depending on debug_prune_btf.
	(btf_early_finsih): New initialization for debug_prune_btf.
	(btf_add_used_type): New function.
	(btf_used_type_list_cb): Likewise.
	(btf_collect_pruned_types): Likewise.
	(btf_add_vars): Handle special case for variables in ".maps" section
	when generating BTF for BPF CO-RE target.
	(btf_late_finish): Use btf_collect_pruned_types when debug_prune_btf
	is in effect.  Move some initialization to btf_early_finish.
	(btf_finalize): Additional deallocation for debug_prune_btf.
	* common.opt (gprune-btf): New flag.
	* ctfc.cc (init_ctf_strtable): Make non-static.
	* ctfc.h (init_ctf_strtable, ctfc_delete_strtab): Make extern.
	* doc/invoke.texi (Debugging Options): Document -gprune-btf.

gcc/testsuite/
	* gcc.dg/debug/btf/btf-prune-1.c: New test.
	* gcc.dg/debug/btf/btf-prune-2.c: Likewise.
	* gcc.dg/debug/btf/btf-prune-3.c: Likewise.
	* gcc.dg/debug/btf/btf-prune-maps.c: Likewise.