Don't claim multilib support.

https://bugs.gentoo.org/675954
--- a/gcc/config/t-musl
+++ b/gcc/config/t-musl
@@ -1,2 +1,3 @@
 MULTIARCH_DIRNAME := $(subst -linux-gnu,-linux-musl,$(MULTIARCH_DIRNAME))
-MULTILIB_OSDIRNAMES := $(subst -linux-gnu,-linux-musl,$(MULTILIB_OSDIRNAMES))
+# No real multilib support. Default to 'lib': https://bugs.gentoo.org/675954
+MULTILIB_OSDIRNAMES :=