mk: fix link with CC

mk: fix link with CC

It appeared in commit 21cdc2e77a4ca999 ("fix 32-bit link with gcc")
that linker options must be prefixed by -Wl, when using CC.
So CPU_LDFLAGS is prefixed in rte.lib.mk.
Then commit 815cfb7925bb6de ("fix link of combined shared library using CC")
introduced another prefixing of CPU_LDFLAGS in rte.sharelib.mk,
included in lib/Makefile.
Because CPU_LDFLAGS is an exported variable, the prefixing is done twice.
Initial patch of commit 815cfb7925bb6de had a workaround but it hasn't
been applied in favor of this proper fix.

Now variables are not overriden when prefixing.

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>