…ig, so that it doesn't break

cross-compilation

Passing swift_core_private_link_libraries to PRIVATE_LINK_LIBRARIES only works either when
building a single host SDK alone or when needed by all SDKs, so move all that configuration
to a swift_core_private_libraries() function that's called from add_swift_target_library()
for each SDK/arch instead. Similarly, call a swift_runtime_static_libraries() function to
split off libraries for the static stdlib with linux. Finally, remove the last uses of
SWIFT_CONFIGURED_SDKS for anything more than checking, using SWIFT_SDKS instead, and move
the static-stdlib linker file generation to the stdlib/ directory.

…ig, so that it doesn't break

cross-compilation

Passing swift_core_private_link_libraries to PRIVATE_LINK_LIBRARIES only works either when
building a single host SDK alone or when needed by all SDKs, so move all that configuration
to a swift_core_private_libraries() function that's called from add_swift_target_library()
for each SDK/arch instead. Also, generate a static-executable-args.lnk for each ELF SDK,
rather than just for linux if it is the host SDK.