libcxx: fix build for x86/64

When building libcxx for x86/64, the library is installed in /usr/lib64.
As the install section tries to copy the library from /usr/lib, this
breaks build on x86/64. Override the lib dir suffix to fix this.

Fixes: 856ea2bad3 ("libcxx: Add package")
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Acked-by: Rosen Penev <rosenp@gmail.com>

package/libs/libcxx/Makefile

@@ -45,6 +45,7 @@ CMAKE_OPTIONS += \
 	-DLIBCXX_INCLUDE_BENCHMARKS=OFF \
 	-DLIBCXX_INCLUDE_DOCS=OFF \
 	-DLIBCXX_INCLUDE_TESTS=OFF \
+	-DLIBCXX_LIBDIR_SUFFIX="" \
 	-DLIBCXX_STANDALONE_BUILD=ON \
 	-DLIBCXX_HAS_MUSL_LIBC=$(if $(CONFIG_USE_MUSL),ON,OFF)