compiler_types.h: Define __retain for __attribute__((__retain__))

Some code includes the __used macro to prevent functions and data from
being optimized out. This macro implements __attribute__((__used__)),
which operates at the compiler and IR-level, and so still allows a linker
to remove objects intended to be kept.

Compilers supporting __attribute__((__retain__)) can address this gap by
setting the flag SHF_GNU_RETAIN on the section of a function/variable,
indicating to the linker the object should be retained. This attribute is
available since gcc 11, clang 13, and binutils 2.36.

Provide a __retain macro implementing __attribute__((__retain__)), whose
first user will be the '__bpf_kfunc' tag.

[ Additional remark from discussion:

  Why is CONFIG_LTO_CLANG added here? The __used macro permits garbage
  collection at section level, so CLANG_LTO_CLANG without
  CONFIG_LD_DEAD_CODE_DATA_ELIMINATION should not change final section
  dynamics?

  The conditional guard was included to ensure consistent behaviour
  between __retain and other features forcing split sections. In
  particular, the same guard is used in vmlinux.lds.h to merge split
  sections where needed. For example, using __retain in LLVM builds
  without CONFIG_LTO was failing CI tests on kernel-patches/bpf because
  the kernel didn't boot properly. And in further testing, the kernel
  had no issues loading BPF kfunc modules with such split sections, so
  the module (partial) linking scripts were left alone. ]

Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/ZlmGoT9KiYLZd91S@krava/T/
Link: https://lore.kernel.org/bpf/b31bca5a5e6765a0f32cc8c19b1d9cdbfaa822b5.1717477560.git.Tony.Ambardar@gmail.com