btf: move pahole check in scripts/link-vmlinux.sh to lib/Kconfig.debug

When DEBUG_INFO_DWARF5 is selected, pahole 1.21+ is required to enable
DEBUG_INFO_BTF.

When DEBUG_INFO_DWARF4 or DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT is selected,
DEBUG_INFO_BTF can be enabled without pahole installed, but a build error
will occur in scripts/link-vmlinux.sh:

    LD      .tmp_vmlinux1
  BTF: .tmp_vmlinux1: pahole (pahole) is not available
  Failed to generate BTF for vmlinux
  Try to disable CONFIG_DEBUG_INFO_BTF

We did not guard DEBUG_INFO_BTF by PAHOLE_VERSION when previously
discussed [1].

However, commit 613fe1692377 ("kbuild: Add CONFIG_PAHOLE_VERSION")
added CONFIG_PAHOLE_VERSION after all. Now several CONFIG options, as
well as the combination of DEBUG_INFO_BTF and DEBUG_INFO_DWARF5, are
guarded by PAHOLE_VERSION.

The remaining compile-time check in scripts/link-vmlinux.sh now appears
to be an awkward inconsistency.

This commit adopts Nathan's original work.

[1]: https://lore.kernel.org/lkml/[email protected]/

Signed-off-by: Masahiro Yamada <[email protected]>
Reviewed-by: Alan Maguire <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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