[AArch64][Fuchsia] Allow -mcmodel=kernel for --target=aarch64-fuchsia

This mode is just like -mcmodel=small except that it moves the
thread pointer from TPIDR_EL0 to TPIDR_EL1.

Patch by Roland McGrath.

Differential Revision: https://reviews.llvm.org/D31624

llvm-svn: 299462