ada: Implement __gnat_get_executable_load_address for Android

Even though Android is a variant of Linux, it doesn't expose
a _r_debug struct so handily as with GLIBC.  Program headers
from dl_iterate_phdr show name/addr sequences of pairs such as

   dpli_name                                       dpli_addr
   /system/bin/linker64                            0x720f3d3000
   /data/data/com.arachnoid.sshelper/home/testone  0x64f217f000 (exe)
   [vdso]                                          0x720f3d2000
   /apex/com.android.runtime/lib64/bionic/libc.so  0x720de25000
   /apex/com.android.runtime/lib64/bionic/libdl.so 0x720ddd6000
   /system/lib64/libnetd_client.so                 0x720dc83000
   /system/lib64/libc++.so                         0x720d30b000
   /apex/com.android.runtime/lib64/bionic/libm.so  0x720dcc3000

The name is not empty for the executable and there's no documentation
saying it would always come second or so, so this provides no obvious
reliable means to discriminate the exe load address.

As a variant of linux, the default link policy for Android authorizes
undefined refs when creating shared libs, with resolution deferred at least
until the lib is loaded. "main" will have to be available then, so resort
to dladdr(&main) to figure out the main module load address from here.

gcc/ada/ChangeLog:

	* adaint.c (__gnat_get_executable_load_address): Provide
	an implementation based on dladdr(&main) and use it for Android.