Calling convention on x64

Calling convention on x64

I saw in several places that Microsoft calling conventions for x64
platforms were narrowed down to just one: Caller cleans stack (cdecl), and
parameters are passed in a combination of stack and registers (I am not
going into the exact details here). I assume that if this is the calling
convention of the OS, then probably all other compilers targeting Windows
(e.g. mingw-w64) follow it, too.
Is this calling convention true also on other major platforms (x64 Linux,
etc.)? Or does Linux still use multiple calling conventions, or even just
one but a different one? Do GCC or Clang allow for multiple calling
conventions on x64 Linux?