configure: Always use AC_LINK_ELSEIF when testing against assembler
This fixes Trac #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 7b090b53fea065d2cfd967ea919426af9ba8d737)