kbuild: use HOSTLDFLAGS for single .c executables

When compiling executables from a single .c file, the linker is also
invoked. Pass the HOSTLDFLAGS like for other linker commands.

Signed-off-by: Robin Jarry <robin.jarry@6wind.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>