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>