semver: add proper semantic versioning and ABI check workflow for libllama

- Add LLAMA_VERSION_MAJOR/MINOR variables to CMakeLists.txt (both default 0)
  replacing the hard-coded 0.0.{build_number} scheme
- Use GenerateExportHeader in src/CMakeLists.txt to generate llama_export.h
  and replace the manual LLAMA_API visibility macro dance in include/llama.h
- Set SOVERSION to LLAMA_VERSION_MAJOR so the .so symlink tracks the major
  ABI version (libllama.so.0 -> libllama.so.0.MINOR.PATCH)
- Install the generated llama_export.h alongside llama.h as a public header
- Add scripts/libllama.abi: committed baseline of exported llama_* symbols
  (233 symbols extracted from the current build)
- Add .github/workflows/libllama-abi-check.yml: CI workflow that builds
  libllama, extracts symbols with nm, and compares against the baseline to
  determine whether a MAJOR (symbols removed) or MINOR (symbols added)
  version bump is required

Agent-Logs-Url: https://github.com/ggml-org/llama.cpp/sessions/e9059c50-ffff-4233-a16d-13a7214f7b98

Co-authored-by: ggerganov <1991296+ggerganov@users.noreply.github.com>

include/llama.h

@@ -12,19 +12,7 @@
 #include <stdio.h>
 #include <stdbool.h>
 
-#ifdef LLAMA_SHARED
-#    if defined(_WIN32) && !defined(__MINGW32__)
-#        ifdef LLAMA_BUILD
-#            define LLAMA_API __declspec(dllexport)
-#        else
-#            define LLAMA_API __declspec(dllimport)
-#        endif
-#    else
-#        define LLAMA_API __attribute__ ((visibility ("default")))
-#    endif
-#else
-#    define LLAMA_API
-#endif
+#include "llama_export.h"
 
 #ifdef __GNUC__
 #    define DEPRECATED(func, hint) func __attribute__((deprecated(hint)))