eclass/usr-ldscript: make gen_usr_ldscript a noop on Prefix

Like in the old toolchain-funcs.eclass, don't do any usr-linking on
Prefix (because there is no usr-split).  In particular, this avoid doing
stuff like calling linkers with arguments they don't support and
therefore dying.

Signed-off-by: Fabian Groffen <grobian@gentoo.org>

eclass/usr-ldscript.eclass

@@ -37,6 +37,7 @@ gen_usr_ldscript() {
 	local lib libdir=$(get_libdir) output_format="" auto=false suffix=$(get_libname)
 
 	tc-is-static-only && return
+	use prefix && return
 
 	# We only care about stuffing / for the native ABI. #479448
 	if [[ $(type -t multilib_is_native_abi) == "function" ]] ; then