[lldb] [debugserver] Simplify handling of arch specific files

There are no duplicates among the include files, and all the
source files are wrapped in architecture ifdefs, so there's no harm
in including all of them, always.

This fixes builds if TARGET_TRIPLE is set to something else than the
build architecture.

This also allows building for multiple architectures at once by
setting CMAKE_OSX_ARCHITECTURES.

Differential Revision: https://reviews.llvm.org/D116625