posix arch: Use zephyr_link_libraries() to set -m32

For some reason, some users have been facing a bizarre issue
in which the -m32 option was not being passed to the linker
by cmake when building for the POSIX arch as a 32bit target,
even though the option was actually supported.

Instead of using zephyr_ld_options() which checks if an
option is supported and drops it otherwise, use
zephyr_link_libraries()

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>