[MSan] Use COMMON_INTERCEPTOR_ENTER in libdl interceptors.

This fixes fails in test/msan/dlerror.cc - when real dlerror calls strcmp,
our strcmp interceptor now skips poison checking, since it's called in
interceptor context.  Strictly speaking, only the dlerror change is
necessary to fix the fail, but let's also change the other two just in case.

Differential Revision: http://reviews.llvm.org/D19499

llvm-svn: 267486