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 <[e-mail]>
Acked-by: Song Liu <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Josh Poimboeuf <[e-mail]>

scripts/livepatch/klp-build

···
 	[[ -v CONFIG_GCC_PLUGIN_RANDSTRUCT ]] &&	\
 		die "kernel option 'CONFIG_GCC_PLUGIN_RANDSTRUCT' not supported"
 
+	[[ -v CONFIG_AS_IS_LLVM ]] &&				\
+		[[ "$CONFIG_AS_VERSION" -lt 200000 ]] &&	\
+		die "Clang assembler version < 20 not supported"
+
 	return 0
 }