kbuild: add KBUILD_HOSTLDFLAGS to cmd_host-csingle

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

cherry-pick kbuild change from Linux:

63185b46cdb3 (kbuild: use HOSTLDFLAGS for single .c executables)

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>