livepatch/klp-build: Require Clang assembler >= 20

livepatch/klp-build: Require Clang assembler >= 20

Some special sections specify their ELF section entsize, for example:

  .pushsection section, "M", @progbits, 8

The entsize (8 in this example) is needed by objtool klp-diff for
extracting individual entries.

Clang assembler versions older than 20 silently ignore the above
construct and set entsize to 0, resulting in the following error:

  .discard.annotate_data: missing special section entsize or annotations

Add a klp-build check to prevent the use of Clang assembler versions
prior to 20.

Fixes: 24ebfcd65a87 ("livepatch/klp-build: Introduce klp-build script for generating livepatch modules")
Reported-by: Song Liu <song@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Link: https://patch.msgid.link/957fd52e375d0e2cfa3ac729160da995084a7f5e.1769562556.git.jpoimboe@kernel.org
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>