mk: fix 32-bit link with gcc

mk: fix 32-bit link with gcc

Some linker options were not prefixed by -Wl, when using CC:
	-z muldefs
	-melf_i386 (CPU_LDFLAGS in 32-bit config)

I didn't see any error with -z muldefs but it isn't documented in gcc
manual. So it's safer to explicitly pass it to the linker.
Also building 32-bit shared library raises this error:
	gcc: error: unrecognized command line option ‘-melf_i386’

Using macro linkerprefix fixes it.

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Reviewed-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>