Rebase gnatdroid to gcc6-aux (6.0), add support for MARSHMELLOW

The gnatdroid cross-compilers (*BSD => Android) were the last two users
of gcc-aux (gcc 4.9 Ada compiler).  They are now based on gcc6-aux.

The Android NDK release 11 just came out with support for Android API 23
(Marshmellow) so now gnatdroid supports it.  I removed API 8 and 9
(Froyo and Gingerbread) from gnatdroid-armv7.  Froyo currently is installed
on less than 0.1% of Android devices, and Gingerbread is dropping under 2%
soon.  Many sysroot distfiles were re-rolled to gain required headers, so
a distfiles subdirectory was added as well.

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