configure: Always use AC_LINK_ELSEIF when testing against assembler

This fixes #16440, where the build system incorrectly concluded that the
`.subsections_via_symbols` assembler directive was supported on a Linux
system. This was caused by the fact that gcc was invoked with `-flto`;
when so-configured gcc does not call the assembler but rather simply
serialises its AST for compilation during the final link.

This is described in Note [autoconf assembler checks and -flto].

(cherry picked from commit 7b090b53)