No libdl.a static library

#### SSIA, there is no static version of the libdl library

This makes static builds which attempt to dynamically open other libraries impossible. Missing libz.a or libm.a could be worked around by using dlopen()/dlsym()/dlclose(), but missing libdl.a _cannot_ be worked around (unless one is an expert with writing one's own dynamic linker).

#### Environment Details

* NDK Version: 15.1.4119039
* Build sytem: ndk-build
* Host OS: Linux
* Compiler: Clang
* NDK API level: All (`find ${NDK_DIR} -name libdl.a` => no results)

Start by inspecting the NDK build and packaging configuration used by ndk-build, then verify the library outputs with the reported find command across API levels. Done means a libdl.a is available for the supported API levels and static builds can use dlopen(), dlsym(), and dlclose().

Written by the indexing model from the issue text.