add visibility=default to exported functions, visibility=hidden to most assembler

[mattip]

Towards #5560

• Add a OPENBLAS_EXPORT macro that maps to __attribute__ ((visibility ("default")))
• Add the macro to all the exported functions: everything in interfaces and some in kernels/generic
• Add a directive to the PROLOGUE assembler macro to hide functions by default, and use a PROLOGUE_EXPORT macro where functions should be exported

Where would be the best place to add a non-MSVC default -fvisibility=hidden? Somewhere in Makefile.system I guess, but how/where?

Note most of this was done as part of MacPython/openblas-libs#241, and I tested it there on the most popular builds.

[mattip]

Rather than add OPENBLAS_EXPORT to all the functions here and in ../utils, I removed the flag from the Makefile. I should do the same for the CMake. How does one tweak the flags per-directory in CMake?

[mattip]

The clang builds are complaining about the assembler directive .hidden

  ../kernel/x86_64/qconjg.S:42:35: error: unknown directive
 .text;.align 5; .globl _qconjg_; .hidden _qconjg_; _qconjg_:
                                  ^

But the passing gcc builds are happy enough with it. I wonder if there is another way to tell clang to hide the assembler function without resorting to objcopy

[mattip]

I got the documentation for .hidden here which mentions ELF. So I will put it behind #ifdef __ELF__

[mattip]

The Windows_cl job is failing since on windows I need the full dllexport, dllimport dance in cblas.h. It is used both internally during compilation (hence dllimport) and externally shipped (hence dllexport).

The other failures are due to timeouts or network issues, and I don't think they are connected to this PR.

I am still thinking about the best way to add -fvisibility=hidden to CFLAGS, any hints are welcome.

[mattip]

Ahh, the shipped cblas.h is actually heavily modified in the build process (in both CMakeLists.txt and Makefile.install, look for "Generating cblas.h"), so I could add the OPENBLAS_EXPORT for the build and remove it in the generated file.