As was already done for HAVE_PTHREAD_(GETNAME/GET_NAME)_NP

Cygwin environment has pthread functionality but LLVM integration
doesn't care it nor provide fallback.
Using Linux integration for Cygwin works fine.

This commit resolves a potential issue of working with uninitialized
memory when querying the CPU's affinity. The man page of
`sched_getaffinity` does not guarantee that the memory will be fully
overwritten, so this change should ensure that issues are avoided.

This PR is in reference to porting LLDB on AIX.
Link to discussions on llvm discourse and github:

1. https://discourse.llvm.org/t/port-lldb-to-ibm-aix/80640
2. https://github.com/llvm/llvm-project/issues/101657
The complete changes for porting are present in this draft PR:
https://github.com/llvm/llvm-project/pull/102601

- Added changes to getProcFile() with threadID, including testcase for
AIX.
- Added support for AIX to get_threadid() from llvm.

This header was included after the implementations to work around an
issue with FreeBSD, however, , this causes some issues when
dllexport\explicit visibility
attributes will be added to the headers on Windows, since the
definitions need to see the declarations for the attributes to apply.

This is part of the work to enable LLVM_BUILD_LLVM_DYLIB and plugins on
windows.

---------

Co-authored-by: Tom Stellard <tstellar@redhat.com>

I'm on a system that has have_pthread_getname_np defined but set to 0.
The correct behavior here is not to use the function, but presently the
macros will attempt to use a non-existing function.

This previously worked before
https://github.com/llvm/llvm-project/pull/106486/files

Haiku has pthread_setname_np() / pthread_getname_np().