configure: Always use AC_LINK_ELSEIF when testing against assembler

This fixes #16440 (closed), 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].