ZGC: Put C2 load barrier stub routines in separate codeblobs

Putting C2 load barrier stub routines in separate blobs make the labels in print assembly show the type of the barrier. This simplifies debugging.

A load barrier slow path (that delegates to the stub routine) will have a stub name that reveals the barrier type, like this:

  0x00007f8fb91b4b58: lea 0x10(%r9),%r11
  0x00007f8fb91b4b5c: callq 0x00007f8fb9009a60 ; {runtime_call zgc_load_barrier_weak_stubs}
  0x00007f8fb91b4b61: jmpq 0x00007f8fb91b4a23