toolchain.eclass: propagate USE=zstd to libgccjit build

You *can* do LTO with libgccjit, apparently, and it even works if we don't
break the compression used for LTO with it. Pass down the zstd arg from
USE=zstd like we do for the regular build.

Thanks to StrawberryTea for reporting.

Closes: https://bugs.gentoo.org/926953
Signed-off-by: Sam James <sam@gentoo.org>

eclass/toolchain.eclass

@@ -1462,7 +1462,7 @@ toolchain_src_configure() {
 			# respect USE=graphite here in case the user passes some
 			# graphite flags rather than try strip them out.
 			$(use_with graphite isl)
-			--without-zstd
+			$(use_with zstd)
 			--with-system-zlib
 		)