Move compat symlinks from makefile to soong

Two APEXes (i18n, art) requires compat symlinks and these symlinks are
created via ld.config.txt module's POST_INSTALL_CMDS.

  $ m installclean
  $ m ld.config.txt
  $ tree $OUT/system

.../system
├── bin
│   ├── dalvikvm -> /apex/com.android.art/bin/dalvikvm
│   └── dex2oat -> /apex/com.android.art/bin/dex2oat
├── etc
│   └── ld.config.txt
└── usr
    └── icu -> /apex/com.android.i18n/etc/icu

By the way, ld.config.txt is going to removed since linkerconfig
generates it on device.

So, we're moving symlink creation from ld.config.txt(rootdir/Android.mk)
to APEXes' POST_INSTALL_CMDS.

  $ m installclean
  $ m com.android.i18n
  $ tree $OUT/system

.../system
├── apex
│   └── com.android.i18n.apex
└── usr
    └── icu -> /apex/com.android.i18n/etc/icu

  $ m installclean
  $ m com.android.art
  $ tree $OUT/system

.../system
├── apex
│   └── com.android.art.debug.apex
├── bin
│   ├── dalvikvm -> /apex/com.android.art/bin/dalvikvm
│   └── dex2oat -> /apex/com.android.art/bin/dex2oat

Bug: 143192278
Test: m && boot (since these two apexes are bootstrap apexes)
Change-Id: Ib29ea9f9ac40e74f78cd530e7daef8c51292fd24