multilib.eclass: multilib_env(): set LIBDIR=lib for *-musl*
In contrast to glibc musl profiles use 'lib' layour for 32-bit
and 64-bit targets. multilib_env() did not take it into account
and assumed glibc's lib64 layout.
That breaks crossdev as it uses multilib_env to extract target
definition. Native builds are unaffected by this change.
Bug: https://bugs.gentoo.org/675954
Bug: https://gcc.gnu.org/PR90077
Bug: https://github.com/gentoo/musl/issues/245
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>