LLVM does not support the __thread attribute for thread
local storage and may generate incorrect code for global
register variables. We want to allow building the runtime with
LLVM-based compilers such as llvm-gcc and clang,
particularly for MacOS.

This patch changes the gct variable used by the garbage
collector to use pthread_getspecific() for thread local
storage when an llvm based compiler is used to build the
runtime.