Merge branch 'libbpf: support custom .rodata.*/.data.* sections'

Andrii Nakryiko says:

====================

This patch set refactors internals of libbpf to enable support for multiple
custom .rodata.* and .data.* sections. Each such section is backed by its own
BPF_MAP_TYPE_ARRAY, memory-mappable just like .rodata/.data. This is not
extended to .bss because .bss is not a great name, it is generated by compiler
with name that reflects completely irrelevant historical implementation
details. Given that users have to annotate their variables with
SEC(".data.my_sec") explicitly, standardizing on .rodata. and .data. prefixes
makes more sense and keeps things simpler.

Additionally, this patch set makes it simpler to work with those special
internal maps by allowing to look them up by their full ELF section name.

Patch #1 is a preparatory patch that deprecates one libbpf API and moves
custom logic into libbpf.c, where it's used. This code is later refactored
with the rest of libbpf.c logic to support multiple data section maps.

See individual patches for all the details.

For new custom "dot maps", their full ELF section names are used as the names
that are sent into the kernel. Object name isn't prepended like for
.data/.rodata/.bss. The reason is that with longer custom names, there isn't
much space left for object name anyways. Also, if BTF is supported,
btf_value_type_id points to DATASEC BTF type, which contains full original ELF
name of the section, so tools like bpftool could use that to recover full
name. This patch set doesn't add this logic yet, this is left for follow up
patches.

One interesting possibility that is now open by these changes is that it's
possible to do:

    bpf_trace_printk("My fmt %s", sizeof("My fmt %s"), "blah");

and it will work as expected. I haven't updated libbpf-provided helpers in
bpf_helpers.h for snprintf, seq_printf, and printk, because using
`static const char ___fmt[] = fmt;` trick is still efficient and doesn't fill
out the buffer at runtime (no copying). But we might consider updating them in
the future, especially with the array check that Kumar proposed (see [0]).

  [0] https://lore.kernel.org/bpf/20211012041524.udytbr2xs5wid6x2@apollo.localdomain/

v1->v2:
  - don't prepend object name for new dot maps;
  - add __read_mostly example in selftests (Daniel).
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>