Don't load libc_malloc_* libs from runtime ns for libc_scudo

malloc_common_dynamic.cpp is compiled into both libc.so and
libc_scudo.so. When compiled for libc_scudo.so, it doesn't try to load
libc_malloc_* libs from the runtime linker namespace. This is because,
unlike libc.so which is shared from the runtime APEX, libc_scudo.so is
copied to any APEX that it needs. Furthermore, libdl_android which
provides android_get_exported_namespace is not available for vendors. So
the vendor variant of libc_scudo.so can't anyway locate the runtime
namespace.

Bug: 130213757
Bug: 122566199
Test: `m libc_scudo libc_scudo` is successful
Test: inspect the built library to see if it has reference to
android_get_exported_namespace

Change-Id: I4c41de361fdb3fa34b95218923f4ce4e9c010f9e