gh-111225: Link extension modules against libpython on Android (#115780)

Part of the work on PEP 738: Adding Android as a supported platform.

* Rename the LIBPYTHON variable to MODULE_LDFLAGS, to more accurately 
  reflect its purpose.
* Edit makesetup to use MODULE_LDFLAGS when linking extension modules.
* Edit the Makefile so that extension modules depend on libpython on 
  Android and Cygwin.
* Restore `-fPIC` on Android. It was removed several years ago with a 
  note that the toolchain used it automatically, but this is no longer
  the case. Omitting it causes all linker commands to fail with an error
  like `relocation R_AARCH64_ADR_PREL_PG_HI21 cannot be used against
  symbol '_Py_FalseStruct'; recompile with -fPIC`.