pppd: Enable plugin support in pppd

This patch makes it possible for pppd to load plugins in Android. Next
changes were made:

1. Link pppd with libdl library, which allows dynamic loading of shared
   libraries.

2. Use -rdynamic to export all pppd symbols in dynamic symbol table,
   so that plugins can link their undefined symbols against pppd.

3. Change plugins search path in pppd to:
   - /system/lib/    (on 32-bit systems)
   - /system/lib64/  (on 64-bit systems)

Last change was done for two reasons:

1. Fix plugin loading on 64-bit architectures

   On ARM64 pppd will try to load 32-bit plugins (.so) files from /lib,
   which leads to this kind of errors:

       E pppd    : dlopen failed:
                   "/system/lib/pppd/2.4.7/pppol2tp-android.so"
                   is 32-bit instead of 64 bit
       E pppd    : Couldn't load plugin pppol2tp-android.so

   To overcome this issue let's make pppd look for plugins in lib/ or
   in lib64/ directory based on __LP64__ definition.

2. Search for plugins in /system/${LIB}

   Bionic loader allows us to dlopen() libraries only from
   /system/${LIB} root directory (as specified in ld.config.txt, see
   "default" linker namespace). When using upstream pppd location, which
   is /system/${LIB}/pppd/2.4.7/, next error occurs in logcat, when pppd
   is trying to load some plugin:

   pppd : dlopen failed: library
          "/system/lib64/pppd/2.4.7/pppol2tp-android.so"
   needed or dlopened by "/system/bin/pppd" is not accessible for the
   namespace "(default)"

   Make pppd look only in /system/${LIB} root directory to fix this
   issue.

Change-Id: I13c8c30337b1307911bdafa711afa1c3c34e99a6
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>