linker: disable ld.config.txt in ASAN mode

Currently, multiple namespaces does not support ASAN mode where some
symbols should be intercepted via LD_PRELOAD; LD_PRELOADed libs are not
preloaded into the linked namespaces other than the default namespace.
Until we fix the problem, we temporarily disable ld.config.txt in ASAN
mode.

Bug: 38114603
Test: 1. ./external/compiler-rt/lib/asan/scripts/asan_device_setup --lib
prebuilts/clang/host/linux-x86/clang-stable/lib64/clang/3.8/lib/linux
2. enable talkback shortcut
3. in the home screen, hold vol-up/down together
4. device does not reboots and talkback shortcut is toggled
Test: bionic-unit-tests and linker-unit-tests successful

Change-Id: I091874dd36b36f56dff2ad899d76ff86931909ed