Load libc_malloc_* libraries from the runtime APEX

/system/lib/libc.so is a symlink to libc.so in the runtime APEX.
libc_malloc_* libraries are bundled with libc.so because they share
implementation details.

However, since libc.so is loaded in the default namespace where the
runtime APEX path (/apex/com.android.runtime/lib) is not accessible,
libc.so has been using libc_malloc_* from /system/lib. This is
wrong because libc.so (from the runtime APEX) and libc_malloc_* (from
the platform) may not be in-sync.

libc.so now uses android_dlopen_ext to load libc_malloc_* libraries
correctly from the "runtime" linker namespace.

Bug: 122566199
Test: bionic-unit-tests

Change-Id: I46980fbe89e93ea79a7760c9b8eb007af0ada8d8
(cherry picked from commit aea5c344230b292c56cf96f77b33f6f35f9b30be)