Symbol list sources

[Po Lu]

Google severally provide lists of symbols exported by libc, libm, libdl,
and related NDK libraries by older Android releases, at this URL, which
might be a welcome resource for Gnulib developers:

  
https://android.googlesource.com/platform/development/+/refs/heads/marshmallow-release/ndk/platforms

in which the number in each subdirectory is the value of __ANDROID_API__
specifying the version pertained to, and symbol lists for libraries on
each supported architecture are saved under "arch-*/symbols."