Small refactoring and cleanup
Make codegen sort callbacks in reverse order to speed up insertion
Add src/sdk/plugin.h to SAMPGDK_SOURCES
Add SAMPGDK_CDECL and SAMPGDK_STDCALL macros
Define AMXAPI as SAMPGDK_EXPORT SAMPGDK_CDECL
Always define OS identification macros

No matter whether you are on Windows or Linux both SAMPGDK_LINUX
and SAMPGDK_WINDOWS are now defined, but only one of them evaluates
to non-zero value depending on the OS.